Add Basilisk

1 files changed, 4929 insertions, 0 deletions

diff --git a/application/basilisk/components/sessionstore/SessionStore.jsm b/application/basilisk/components/sessionstore/SessionStore.jsm
new file mode 100644
index 000000000..c583d534d
--- /dev/null
+++ b/application/basilisk/components/sessionstore/SessionStore.jsm
@@ -0,0 +1,4929 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+this.EXPORTED_SYMBOLS = ["SessionStore"];
+
+const Cu = Components.utils;
+const Cc = Components.classes;
+const Ci = Components.interfaces;
+const Cr = Components.results;
+
+// Current version of the format used by Session Restore.
+const FORMAT_VERSION = 1;
+
+const TAB_STATE_NEEDS_RESTORE = 1;
+const TAB_STATE_RESTORING = 2;
+const TAB_STATE_WILL_RESTORE = 3;
+
+// A new window has just been restored. At this stage, tabs are generally
+// not restored.
+const NOTIFY_SINGLE_WINDOW_RESTORED = "sessionstore-single-window-restored";
+const NOTIFY_WINDOWS_RESTORED = "sessionstore-windows-restored";
+const NOTIFY_BROWSER_STATE_RESTORED = "sessionstore-browser-state-restored";
+const NOTIFY_LAST_SESSION_CLEARED = "sessionstore-last-session-cleared";
+const NOTIFY_RESTORING_ON_STARTUP = "sessionstore-restoring-on-startup";
+const NOTIFY_INITIATING_MANUAL_RESTORE = "sessionstore-initiating-manual-restore";
+const NOTIFY_CLOSED_OBJECTS_CHANGED = "sessionstore-closed-objects-changed";
+
+const NOTIFY_TAB_RESTORED = "sessionstore-debug-tab-restored"; // WARNING: debug-only
+
+// Maximum number of tabs to restore simultaneously. Previously controlled by
+// the browser.sessionstore.max_concurrent_tabs pref.
+const MAX_CONCURRENT_TAB_RESTORES = 3;
+
+// Amount (in CSS px) by which we allow window edges to be off-screen
+// when restoring a window, before we override the saved position to
+// pull the window back within the available screen area.
+const SCREEN_EDGE_SLOP = 8;
+
+// global notifications observed
+const OBSERVING = [
+  "browser-window-before-show", "domwindowclosed",
+  "quit-application-granted", "browser-lastwindow-close-granted",
+  "quit-application", "browser:purge-session-history",
+  "browser:purge-domain-data",
+  "idle-daily",
+];
+
+// XUL Window properties to (re)store
+// Restored in restoreDimensions()
+const WINDOW_ATTRIBUTES = ["width", "height", "screenX", "screenY", "sizemode"];
+
+// Hideable window features to (re)store
+// Restored in restoreWindowFeatures()
+const WINDOW_HIDEABLE_FEATURES = [
+  "menubar", "toolbar", "locationbar", "personalbar", "statusbar", "scrollbars"
+];
+
+// Messages that will be received via the Frame Message Manager.
+const MESSAGES = [
+  // The content script sends us data that has been invalidated and needs to
+  // be saved to disk.
+  "SessionStore:update",
+
+  // The restoreHistory code has run. This is a good time to run SSTabRestoring.
+  "SessionStore:restoreHistoryComplete",
+
+  // The load for the restoring tab has begun. We update the URL bar at this
+  // time; if we did it before, the load would overwrite it.
+  "SessionStore:restoreTabContentStarted",
+
+  // All network loads for a restoring tab are done, so we should
+  // consider restoring another tab in the queue. The document has
+  // been restored, and forms have been filled. We trigger
+  // SSTabRestored at this time.
+  "SessionStore:restoreTabContentComplete",
+
+  // A crashed tab was revived by navigating to a different page. Remove its
+  // browser from the list of crashed browsers to stop ignoring its messages.
+  "SessionStore:crashedTabRevived",
+
+  // The content script encountered an error.
+  "SessionStore:error",
+];
+
+// The list of messages we accept from <xul:browser>s that have no tab
+// assigned, or whose windows have gone away. Those are for example the
+// ones that preload about:newtab pages, or from browsers where the window
+// has just been closed.
+const NOTAB_MESSAGES = new Set([
+  // For a description see above.
+  "SessionStore:crashedTabRevived",
+
+  // For a description see above.
+  "SessionStore:update",
+
+  // For a description see above.
+  "SessionStore:error",
+]);
+
+// The list of messages we accept without an "epoch" parameter.
+// See getCurrentEpoch() and friends to find out what an "epoch" is.
+const NOEPOCH_MESSAGES = new Set([
+  // For a description see above.
+  "SessionStore:crashedTabRevived",
+
+  // For a description see above.
+  "SessionStore:error",
+]);
+
+// The list of messages we want to receive even during the short period after a
+// frame has been removed from the DOM and before its frame script has finished
+// unloading.
+const CLOSED_MESSAGES = new Set([
+  // For a description see above.
+  "SessionStore:crashedTabRevived",
+
+  // For a description see above.
+  "SessionStore:update",
+
+  // For a description see above.
+  "SessionStore:error",
+]);
+
+// These are tab events that we listen to.
+const TAB_EVENTS = [
+  "TabOpen", "TabBrowserInserted", "TabClose", "TabSelect", "TabShow", "TabHide", "TabPinned",
+  "TabUnpinned"
+];
+
+const NS_XUL = "http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul";
+
+Cu.import("resource://gre/modules/PrivateBrowsingUtils.jsm", this);
+Cu.import("resource://gre/modules/Promise.jsm", this);
+Cu.import("resource://gre/modules/Services.jsm", this);
+Cu.import("resource://gre/modules/Task.jsm", this);
+Cu.import("resource://gre/modules/TelemetryStopwatch.jsm", this);
+Cu.import("resource://gre/modules/TelemetryTimestamps.jsm", this);
+Cu.import("resource://gre/modules/Timer.jsm", this);
+Cu.import("resource://gre/modules/XPCOMUtils.jsm", this);
+Cu.import("resource://gre/modules/debug.js", this);
+Cu.import("resource://gre/modules/osfile.jsm", this);
+
+XPCOMUtils.defineLazyServiceGetter(this, "gSessionStartup",
+  "@mozilla.org/browser/sessionstartup;1", "nsISessionStartup");
+XPCOMUtils.defineLazyServiceGetter(this, "gScreenManager",
+  "@mozilla.org/gfx/screenmanager;1", "nsIScreenManager");
+XPCOMUtils.defineLazyServiceGetter(this, "Telemetry",
+  "@mozilla.org/base/telemetry;1", "nsITelemetry");
+XPCOMUtils.defineLazyModuleGetter(this, "console",
+  "resource://gre/modules/Console.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "RecentWindow",
+  "resource:///modules/RecentWindow.jsm");
+
+XPCOMUtils.defineLazyModuleGetter(this, "GlobalState",
+  "resource:///modules/sessionstore/GlobalState.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "PrivacyFilter",
+  "resource:///modules/sessionstore/PrivacyFilter.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "RunState",
+  "resource:///modules/sessionstore/RunState.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "ScratchpadManager",
+  "resource://devtools/client/scratchpad/scratchpad-manager.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "SessionSaver",
+  "resource:///modules/sessionstore/SessionSaver.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "SessionCookies",
+  "resource:///modules/sessionstore/SessionCookies.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "SessionFile",
+  "resource:///modules/sessionstore/SessionFile.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "setTimeout",
+  "resource://gre/modules/Timer.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "TabAttributes",
+  "resource:///modules/sessionstore/TabAttributes.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "TabCrashHandler",
+  "resource:///modules/ContentCrashHandlers.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "TabState",
+  "resource:///modules/sessionstore/TabState.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "TabStateCache",
+  "resource:///modules/sessionstore/TabStateCache.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "TabStateFlusher",
+  "resource:///modules/sessionstore/TabStateFlusher.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "Utils",
+  "resource://gre/modules/sessionstore/Utils.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "ViewSourceBrowser",
+  "resource://gre/modules/ViewSourceBrowser.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "AsyncShutdown",
+  "resource://gre/modules/AsyncShutdown.jsm");
+
+Object.defineProperty(this, "HUDService", {
+  get: function HUDService_getter() {
+    let devtools = Cu.import("resource://devtools/shared/Loader.jsm", {}).devtools;
+    return devtools.require("devtools/client/webconsole/hudservice").HUDService;
+  },
+  configurable: true,
+  enumerable: true
+});
+
+/**
+ * |true| if we are in debug mode, |false| otherwise.
+ * Debug mode is controlled by preference browser.sessionstore.debug
+ */
+var gDebuggingEnabled = false;
+function debug(aMsg) {
+  if (gDebuggingEnabled) {
+    aMsg = ("SessionStore: " + aMsg).replace(/\S{80}/g, "$&\n");
+    Services.console.logStringMessage(aMsg);
+  }
+}
+
+this.SessionStore = {
+  get promiseInitialized() {
+    return SessionStoreInternal.promiseInitialized;
+  },
+
+  get canRestoreLastSession() {
+    return SessionStoreInternal.canRestoreLastSession;
+  },
+
+  set canRestoreLastSession(val) {
+    SessionStoreInternal.canRestoreLastSession = val;
+  },
+
+  get lastClosedObjectType() {
+    return SessionStoreInternal.lastClosedObjectType;
+  },
+
+  init: function ss_init() {
+    SessionStoreInternal.init();
+  },
+
+  getBrowserState: function ss_getBrowserState() {
+    return SessionStoreInternal.getBrowserState();
+  },
+
+  setBrowserState: function ss_setBrowserState(aState) {
+    SessionStoreInternal.setBrowserState(aState);
+  },
+
+  getWindowState: function ss_getWindowState(aWindow) {
+    return SessionStoreInternal.getWindowState(aWindow);
+  },
+
+  setWindowState: function ss_setWindowState(aWindow, aState, aOverwrite) {
+    SessionStoreInternal.setWindowState(aWindow, aState, aOverwrite);
+  },
+
+  getTabState: function ss_getTabState(aTab) {
+    return SessionStoreInternal.getTabState(aTab);
+  },
+
+  setTabState: function ss_setTabState(aTab, aState) {
+    SessionStoreInternal.setTabState(aTab, aState);
+  },
+
+  duplicateTab: function ss_duplicateTab(aWindow, aTab, aDelta = 0, aRestoreImmediately = true) {
+    return SessionStoreInternal.duplicateTab(aWindow, aTab, aDelta, aRestoreImmediately);
+  },
+
+  getClosedTabCount: function ss_getClosedTabCount(aWindow) {
+    return SessionStoreInternal.getClosedTabCount(aWindow);
+  },
+
+  getClosedTabData: function ss_getClosedTabData(aWindow, aAsString = true) {
+    return SessionStoreInternal.getClosedTabData(aWindow, aAsString);
+  },
+
+  undoCloseTab: function ss_undoCloseTab(aWindow, aIndex) {
+    return SessionStoreInternal.undoCloseTab(aWindow, aIndex);
+  },
+
+  forgetClosedTab: function ss_forgetClosedTab(aWindow, aIndex) {
+    return SessionStoreInternal.forgetClosedTab(aWindow, aIndex);
+  },
+
+  getClosedWindowCount: function ss_getClosedWindowCount() {
+    return SessionStoreInternal.getClosedWindowCount();
+  },
+
+  getClosedWindowData: function ss_getClosedWindowData(aAsString = true) {
+    return SessionStoreInternal.getClosedWindowData(aAsString);
+  },
+
+  undoCloseWindow: function ss_undoCloseWindow(aIndex) {
+    return SessionStoreInternal.undoCloseWindow(aIndex);
+  },
+
+  forgetClosedWindow: function ss_forgetClosedWindow(aIndex) {
+    return SessionStoreInternal.forgetClosedWindow(aIndex);
+  },
+
+  getWindowValue: function ss_getWindowValue(aWindow, aKey) {
+    return SessionStoreInternal.getWindowValue(aWindow, aKey);
+  },
+
+  setWindowValue: function ss_setWindowValue(aWindow, aKey, aStringValue) {
+    SessionStoreInternal.setWindowValue(aWindow, aKey, aStringValue);
+  },
+
+  deleteWindowValue: function ss_deleteWindowValue(aWindow, aKey) {
+    SessionStoreInternal.deleteWindowValue(aWindow, aKey);
+  },
+
+  getTabValue: function ss_getTabValue(aTab, aKey) {
+    return SessionStoreInternal.getTabValue(aTab, aKey);
+  },
+
+  setTabValue: function ss_setTabValue(aTab, aKey, aStringValue) {
+    SessionStoreInternal.setTabValue(aTab, aKey, aStringValue);
+  },
+
+  deleteTabValue: function ss_deleteTabValue(aTab, aKey) {
+    SessionStoreInternal.deleteTabValue(aTab, aKey);
+  },
+
+  getGlobalValue: function ss_getGlobalValue(aKey) {
+    return SessionStoreInternal.getGlobalValue(aKey);
+  },
+
+  setGlobalValue: function ss_setGlobalValue(aKey, aStringValue) {
+    SessionStoreInternal.setGlobalValue(aKey, aStringValue);
+  },
+
+  deleteGlobalValue: function ss_deleteGlobalValue(aKey) {
+    SessionStoreInternal.deleteGlobalValue(aKey);
+  },
+
+  persistTabAttribute: function ss_persistTabAttribute(aName) {
+    SessionStoreInternal.persistTabAttribute(aName);
+  },
+
+  restoreLastSession: function ss_restoreLastSession() {
+    SessionStoreInternal.restoreLastSession();
+  },
+
+  getCurrentState: function (aUpdateAll) {
+    return SessionStoreInternal.getCurrentState(aUpdateAll);
+  },
+
+  reviveCrashedTab(aTab) {
+    return SessionStoreInternal.reviveCrashedTab(aTab);
+  },
+
+  reviveAllCrashedTabs() {
+    return SessionStoreInternal.reviveAllCrashedTabs();
+  },
+
+  navigateAndRestore(tab, loadArguments, historyIndex) {
+    return SessionStoreInternal.navigateAndRestore(tab, loadArguments, historyIndex);
+  },
+
+  getSessionHistory(tab, updatedCallback) {
+    return SessionStoreInternal.getSessionHistory(tab, updatedCallback);
+  },
+
+  undoCloseById(aClosedId) {
+    return SessionStoreInternal.undoCloseById(aClosedId);
+  },
+
+  /**
+   * Determines whether the passed version number is compatible with
+   * the current version number of the SessionStore.
+   *
+   * @param version The format and version of the file, as an array, e.g.
+   * ["sessionrestore", 1]
+   */
+  isFormatVersionCompatible(version) {
+    if (!version) {
+      return false;
+    }
+    if (!Array.isArray(version)) {
+      // Improper format.
+      return false;
+    }
+    if (version[0] != "sessionrestore") {
+      // Not a Session Restore file.
+      return false;
+    }
+    let number = Number.parseFloat(version[1]);
+    if (Number.isNaN(number)) {
+      return false;
+    }
+    return number <= FORMAT_VERSION;
+  },
+
+  /**
+   * Filters out not worth-saving tabs from a given browser state object.
+   *
+   * @param aState (object)
+   *        The browser state for which we remove worth-saving tabs.
+   *        The given object will be modified.
+   */
+  keepOnlyWorthSavingTabs: function (aState) {
+    for (let i = aState.windows.length - 1; i >= 0; i--) {
+      let win = aState.windows[i];
+      for (let j = win.tabs.length - 1; j >= 0; j--) {
+        let tab = win.tabs[j];
+        if (!SessionStoreInternal._shouldSaveTab(tab)) {
+          win.tabs.splice(j, 1);
+          if (win.selected > j) {
+            win.selected--;
+          }
+        }
+      }
+      if (!win.tabs.length) {
+        aState.windows.splice(i, 1);
+        if (aState.selectedWindow > i) {
+          aState.selectedWindow--;
+        }
+      }
+    }
+  },
+};
+
+// Freeze the SessionStore object. We don't want anyone to modify it.
+Object.freeze(SessionStore);
+
+var SessionStoreInternal = {
+  QueryInterface: XPCOMUtils.generateQI([
+    Ci.nsIDOMEventListener,
+    Ci.nsIObserver,
+    Ci.nsISupportsWeakReference
+  ]),
+
+  _globalState: new GlobalState(),
+
+  // A counter to be used to generate a unique ID for each closed tab or window.
+  _nextClosedId: 0,
+
+  // During the initial restore and setBrowserState calls tracks the number of
+  // windows yet to be restored
+  _restoreCount: -1,
+
+  // For each <browser> element, records the current epoch.
+  _browserEpochs: new WeakMap(),
+
+  // Any browsers that fires the oop-browser-crashed event gets stored in
+  // here - that way we know which browsers to ignore messages from (until
+  // they get restored).
+  _crashedBrowsers: new WeakSet(),
+
+  // A map (xul:browser -> nsIFrameLoader) that maps a browser to the last
+  // associated frameLoader we heard about.
+  _lastKnownFrameLoader: new WeakMap(),
+
+  // A map (xul:browser -> object) that maps a browser associated with a
+  // recently closed tab to all its necessary state information we need to
+  // properly handle final update message.
+  _closedTabs: new WeakMap(),
+
+  // A map (xul:browser -> object) that maps a browser associated with a
+  // recently closed tab due to a window closure to the tab state information
+  // that is being stored in _closedWindows for that tab.
+  _closedWindowTabs: new WeakMap(),
+
+  // A set of window data that has the potential to be saved in the _closedWindows
+  // array for the session. We will remove window data from this set whenever
+  // forgetClosedWindow is called for the window, or when session history is
+  // purged, so that we don't accidentally save that data after the flush has
+  // completed. Closed tabs use a more complicated mechanism for this particular
+  // problem. When forgetClosedTab is called, the browser is removed from the
+  // _closedTabs map, so its data is not recorded. In the purge history case,
+  // the closedTabs array per window is overwritten so that once the flush is
+  // complete, the tab would only ever add itself to an array that SessionStore
+  // no longer cares about. Bug 1230636 has been filed to make the tab case
+  // work more like the window case, which is more explicit, and easier to
+  // reason about.
+  _saveableClosedWindowData: new WeakSet(),
+
+  // A map (xul:browser -> object) that maps a browser that is switching
+  // remoteness via navigateAndRestore, to the loadArguments that were
+  // most recently passed when calling navigateAndRestore.
+  _remotenessChangingBrowsers: new WeakMap(),
+
+  // whether a setBrowserState call is in progress
+  _browserSetState: false,
+
+  // time in milliseconds when the session was started (saved across sessions),
+  // defaults to now if no session was restored or timestamp doesn't exist
+  _sessionStartTime: Date.now(),
+
+  // states for all currently opened windows
+  _windows: {},
+
+  // counter for creating unique window IDs
+  _nextWindowID: 0,
+
+  // states for all recently closed windows
+  _closedWindows: [],
+
+  // collection of session states yet to be restored
+  _statesToRestore: {},
+
+  // counts the number of crashes since the last clean start
+  _recentCrashes: 0,
+
+  // whether the last window was closed and should be restored
+  _restoreLastWindow: false,
+
+  // number of tabs currently restoring
+  _tabsRestoringCount: 0,
+
+  // When starting Firefox with a single private window, this is the place
+  // where we keep the session we actually wanted to restore in case the user
+  // decides to later open a non-private window as well.
+  _deferredInitialState: null,
+
+  // Keeps track of whether a notification needs to be sent that closed objects have changed.
+  _closedObjectsChanged: false,
+
+  // A promise resolved once initialization is complete
+  _deferredInitialized: (function () {
+    let deferred = {};
+
+    deferred.promise = new Promise((resolve, reject) => {
+      deferred.resolve = resolve;
+      deferred.reject = reject;
+    });
+
+    return deferred;
+  })(),
+
+  // Whether session has been initialized
+  _sessionInitialized: false,
+
+  // Promise that is resolved when we're ready to initialize
+  // and restore the session.
+  _promiseReadyForInitialization: null,
+
+  // Keep busy state counters per window.
+  _windowBusyStates: new WeakMap(),
+
+  /**
+   * A promise fulfilled once initialization is complete.
+   */
+  get promiseInitialized() {
+    return this._deferredInitialized.promise;
+  },
+
+  get canRestoreLastSession() {
+    return LastSession.canRestore;
+  },
+
+  set canRestoreLastSession(val) {
+    // Cheat a bit; only allow false.
+    if (!val) {
+      LastSession.clear();
+    }
+  },
+
+  /**
+   * Returns a string describing the last closed object, either "tab" or "window".
+   *
+   * This was added to support the sessions.restore WebExtensions API.
+   */
+  get lastClosedObjectType() {
+    if (this._closedWindows.length) {
+      // Since there are closed windows, we need to check if there's a closed tab
+      // in one of the currently open windows that was closed after the
+      // last-closed window.
+      let tabTimestamps = [];
+      let windowsEnum = Services.wm.getEnumerator("navigator:browser");
+      while (windowsEnum.hasMoreElements()) {
+        let window = windowsEnum.getNext();
+        let windowState = this._windows[window.__SSi];
+        if (windowState && windowState._closedTabs[0]) {
+          tabTimestamps.push(windowState._closedTabs[0].closedAt);
+        }
+      }
+      if (!tabTimestamps.length ||
+          (tabTimestamps.sort((a, b) => b - a)[0] < this._closedWindows[0].closedAt)) {
+        return "window";
+      }
+    }
+    return "tab";
+  },
+
+  /**
+   * Initialize the sessionstore service.
+   */
+  init: function () {
+    if (this._initialized) {
+      throw new Error("SessionStore.init() must only be called once!");
+    }
+
+    TelemetryTimestamps.add("sessionRestoreInitialized");
+    OBSERVING.forEach(function(aTopic) {
+      Services.obs.addObserver(this, aTopic, true);
+    }, this);
+
+    this._initPrefs();
+    this._initialized = true;
+  },
+
+  /**
+   * Initialize the session using the state provided by SessionStartup
+   */
+  initSession: function () {
+    TelemetryStopwatch.start("FX_SESSION_RESTORE_STARTUP_INIT_SESSION_MS");
+    let state;
+    let ss = gSessionStartup;
+
+    if (ss.doRestore() ||
+        ss.sessionType == Ci.nsISessionStartup.DEFER_SESSION) {
+      state = ss.state;
+    }
+
+    if (state) {
+      try {
+        // If we're doing a DEFERRED session, then we want to pull pinned tabs
+        // out so they can be restored.
+        if (ss.sessionType == Ci.nsISessionStartup.DEFER_SESSION) {
+          let [iniState, remainingState] = this._prepDataForDeferredRestore(state);
+          // If we have a iniState with windows, that means that we have windows
+          // with app tabs to restore.
+          if (iniState.windows.length)
+            state = iniState;
+          else
+            state = null;
+
+          if (remainingState.windows.length) {
+            LastSession.setState(remainingState);
+          }
+        }
+        else {
+          // Get the last deferred session in case the user still wants to
+          // restore it
+          LastSession.setState(state.lastSessionState);
+
+          if (ss.previousSessionCrashed) {
+            this._recentCrashes = (state.session &&
+                                   state.session.recentCrashes || 0) + 1;
+
+            if (this._needsRestorePage(state, this._recentCrashes)) {
+              // replace the crashed session with a restore-page-only session
+              let url = "about:sessionrestore";
+              let formdata = {id: {sessionData: state}, url};
+              let entry = {url, triggeringPrincipal_base64: Utils.SERIALIZED_SYSTEMPRINCIPAL };
+              state = { windows: [{ tabs: [{ entries: [entry], formdata }] }] };
+            } else if (this._hasSingleTabWithURL(state.windows,
+                                                 "about:welcomeback")) {
+              // On a single about:welcomeback URL that crashed, replace about:welcomeback
+              // with about:sessionrestore, to make clear to the user that we crashed.
+              state.windows[0].tabs[0].entries[0].url = "about:sessionrestore";
+              state.windows[0].tabs[0].entries[0].triggeringPrincipal_base64 = Utils.SERIALIZED_SYSTEMPRINCIPAL;
+            }
+          }
+
+          // Update the session start time using the restored session state.
+          this._updateSessionStartTime(state);
+
+          // make sure that at least the first window doesn't have anything hidden
+          delete state.windows[0].hidden;
+          // Since nothing is hidden in the first window, it cannot be a popup
+          delete state.windows[0].isPopup;
+          // We don't want to minimize and then open a window at startup.
+          if (state.windows[0].sizemode == "minimized")
+            state.windows[0].sizemode = "normal";
+          // clear any lastSessionWindowID attributes since those don't matter
+          // during normal restore
+          state.windows.forEach(function(aWindow) {
+            delete aWindow.__lastSessionWindowID;
+          });
+        }
+      }
+      catch (ex) { debug("The session file is invalid: " + ex); }
+    }
+
+    // at this point, we've as good as resumed the session, so we can
+    // clear the resume_session_once flag, if it's set
+    if (!RunState.isQuitting &&
+        this._prefBranch.getBoolPref("sessionstore.resume_session_once"))
+      this._prefBranch.setBoolPref("sessionstore.resume_session_once", false);
+
+    TelemetryStopwatch.finish("FX_SESSION_RESTORE_STARTUP_INIT_SESSION_MS");
+    return state;
+  },
+
+  _initPrefs : function() {
+    this._prefBranch = Services.prefs.getBranch("browser.");
+
+    gDebuggingEnabled = this._prefBranch.getBoolPref("sessionstore.debug");
+
+    Services.prefs.addObserver("browser.sessionstore.debug", () => {
+      gDebuggingEnabled = this._prefBranch.getBoolPref("sessionstore.debug");
+    }, false);
+
+    this._max_tabs_undo = this._prefBranch.getIntPref("sessionstore.max_tabs_undo");
+    this._prefBranch.addObserver("sessionstore.max_tabs_undo", this, true);
+
+    this._max_windows_undo = this._prefBranch.getIntPref("sessionstore.max_windows_undo");
+    this._prefBranch.addObserver("sessionstore.max_windows_undo", this, true);
+  },
+
+  /**
+   * Called on application shutdown, after notifications:
+   * quit-application-granted, quit-application
+   */
+  _uninit: function ssi_uninit() {
+    if (!this._initialized) {
+      throw new Error("SessionStore is not initialized.");
+    }
+
+    // Prepare to close the session file and write the last state.
+    RunState.setClosing();
+
+    // save all data for session resuming
+    if (this._sessionInitialized) {
+      SessionSaver.run();
+    }
+
+    // clear out priority queue in case it's still holding refs
+    TabRestoreQueue.reset();
+
+    // Make sure to cancel pending saves.
+    SessionSaver.cancel();
+  },
+
+  /**
+   * Handle notifications
+   */
+  observe: function ssi_observe(aSubject, aTopic, aData) {
+    switch (aTopic) {
+      case "browser-window-before-show": // catch new windows
+        this.onBeforeBrowserWindowShown(aSubject);
+        break;
+      case "domwindowclosed": // catch closed windows
+        this.onClose(aSubject).then(() => {
+          this._notifyOfClosedObjectsChange();
+        });
+        break;
+      case "quit-application-granted":
+        let syncShutdown = aData == "syncShutdown";
+        this.onQuitApplicationGranted(syncShutdown);
+        break;
+      case "browser-lastwindow-close-granted":
+        this.onLastWindowCloseGranted();
+        break;
+      case "quit-application":
+        this.onQuitApplication(aData);
+        break;
+      case "browser:purge-session-history": // catch sanitization
+        this.onPurgeSessionHistory();
+        this._notifyOfClosedObjectsChange();
+        break;
+      case "browser:purge-domain-data":
+        this.onPurgeDomainData(aData);
+        this._notifyOfClosedObjectsChange();
+        break;
+      case "nsPref:changed": // catch pref changes
+        this.onPrefChange(aData);
+        this._notifyOfClosedObjectsChange();
+        break;
+      case "idle-daily":
+        this.onIdleDaily();
+        this._notifyOfClosedObjectsChange();
+        break;
+    }
+  },
+
+  /**
+   * This method handles incoming messages sent by the session store content
+   * script via the Frame Message Manager or Parent Process Message Manager,
+   * and thus enables communication with OOP tabs.
+   */
+  receiveMessage(aMessage) {
+    // If we got here, that means we're dealing with a frame message
+    // manager message, so the target will be a <xul:browser>.
+    var browser = aMessage.target;
+    let win = browser.ownerGlobal;
+    let tab = win ? win.gBrowser.getTabForBrowser(browser) : null;
+
+    // Ensure we receive only specific messages from <xul:browser>s that
+    // have no tab or window assigned, e.g. the ones that preload
+    // about:newtab pages, or windows that have closed.
+    if (!tab && !NOTAB_MESSAGES.has(aMessage.name)) {
+      throw new Error(`received unexpected message '${aMessage.name}' ` +
+                      `from a browser that has no tab or window`);
+    }
+
+    let data = aMessage.data || {};
+    let hasEpoch = data.hasOwnProperty("epoch");
+
+    // Most messages sent by frame scripts require to pass an epoch.
+    if (!hasEpoch && !NOEPOCH_MESSAGES.has(aMessage.name)) {
+      throw new Error(`received message '${aMessage.name}' without an epoch`);
+    }
+
+    // Ignore messages from previous epochs.
+    if (hasEpoch && !this.isCurrentEpoch(browser, data.epoch)) {
+      return;
+    }
+
+    switch (aMessage.name) {
+      case "SessionStore:update":
+        // |browser.frameLoader| might be empty if the browser was already
+        // destroyed and its tab removed. In that case we still have the last
+        // frameLoader we know about to compare.
+        let frameLoader = browser.frameLoader ||
+                          this._lastKnownFrameLoader.get(browser.permanentKey);
+
+        // If the message isn't targeting the latest frameLoader discard it.
+        if (frameLoader != aMessage.targetFrameLoader) {
+          return;
+        }
+
+        if (aMessage.data.isFinal) {
+          // If this the final message we need to resolve all pending flush
+          // requests for the given browser as they might have been sent too
+          // late and will never respond. If they have been sent shortly after
+          // switching a browser's remoteness there isn't too much data to skip.
+          TabStateFlusher.resolveAll(browser);
+        } else if (aMessage.data.flushID) {
+          // This is an update kicked off by an async flush request. Notify the
+          // TabStateFlusher so that it can finish the request and notify its
+          // consumer that's waiting for the flush to be done.
+          TabStateFlusher.resolve(browser, aMessage.data.flushID);
+        }
+
+        // Ignore messages from <browser> elements that have crashed
+        // and not yet been revived.
+        if (this._crashedBrowsers.has(browser.permanentKey)) {
+          return;
+        }
+
+        // Record telemetry measurements done in the child and update the tab's
+        // cached state. Mark the window as dirty and trigger a delayed write.
+        this.recordTelemetry(aMessage.data.telemetry);
+        TabState.update(browser, aMessage.data);
+        this.saveStateDelayed(win);
+
+        // Handle any updates sent by the child after the tab was closed. This
+        // might be the final update as sent by the "unload" handler but also
+        // any async update message that was sent before the child unloaded.
+        if (this._closedTabs.has(browser.permanentKey)) {
+          let {closedTabs, tabData} = this._closedTabs.get(browser.permanentKey);
+
+          // Update the closed tab's state. This will be reflected in its
+          // window's list of closed tabs as that refers to the same object.
+          TabState.copyFromCache(browser, tabData.state);
+
+          // Is this the tab's final message?
+          if (aMessage.data.isFinal) {
+            // We expect no further updates.
+            this._closedTabs.delete(browser.permanentKey);
+            // The tab state no longer needs this reference.
+            delete tabData.permanentKey;
+
+            // Determine whether the tab state is worth saving.
+            let shouldSave = this._shouldSaveTabState(tabData.state);
+            let index = closedTabs.indexOf(tabData);
+
+            if (shouldSave && index == -1) {
+              // If the tab state is worth saving and we didn't push it onto
+              // the list of closed tabs when it was closed (because we deemed
+              // the state not worth saving) then add it to the window's list
+              // of closed tabs now.
+              this.saveClosedTabData(closedTabs, tabData);
+            } else if (!shouldSave && index > -1) {
+              // Remove from the list of closed tabs. The update messages sent
+              // after the tab was closed changed enough state so that we no
+              // longer consider its data interesting enough to keep around.
+              this.removeClosedTabData(closedTabs, index);
+            }
+          }
+        }
+        break;
+      case "SessionStore:restoreHistoryComplete":
+        // Notify the tabbrowser that the tab chrome has been restored.
+        let tabData = TabState.collect(tab);
+
+        // wall-paper fix for bug 439675: make sure that the URL to be loaded
+        // is always visible in the address bar if no other value is present
+        let activePageData = tabData.entries[tabData.index - 1] || null;
+        let uri = activePageData ? activePageData.url || null : null;
+        // NB: we won't set initial URIs (about:home, about:newtab, etc.) here
+        // because their load will not normally trigger a location bar clearing
+        // when they finish loading (to avoid race conditions where we then
+        // clear user input instead), so we shouldn't set them here either.
+        // They also don't fall under the issues in bug 439675 where user input
+        // needs to be preserved if the load doesn't succeed.
+        // We also don't do this for remoteness updates, where it should not
+        // be necessary.
+        if (!browser.userTypedValue && uri && !data.isRemotenessUpdate &&
+            !win.gInitialPages.includes(uri)) {
+          browser.userTypedValue = uri;
+        }
+
+        // If the page has a title, set it.
+        if (activePageData) {
+          if (activePageData.title) {
+            tab.label = activePageData.title;
+          } else if (activePageData.url != "about:blank") {
+            tab.label = activePageData.url;
+          }
+        } else if (tab.hasAttribute("customizemode")) {
+          win.gCustomizeMode.setTab(tab);
+        }
+
+        // Restore the tab icon.
+        if ("image" in tabData) {
+          // Use the serialized contentPrincipal with the new icon load.
+          let loadingPrincipal = Utils.deserializePrincipal(tabData.iconLoadingPrincipal);
+          win.gBrowser.setIcon(tab, tabData.image, loadingPrincipal);
+          TabStateCache.update(browser, { image: null, iconLoadingPrincipal: null });
+        }
+
+        let event = win.document.createEvent("Events");
+        event.initEvent("SSTabRestoring", true, false);
+        tab.dispatchEvent(event);
+        break;
+      case "SessionStore:restoreTabContentStarted":
+        if (browser.__SS_restoreState == TAB_STATE_NEEDS_RESTORE) {
+          // If a load not initiated by sessionstore was started in a
+          // previously pending tab. Mark the tab as no longer pending.
+          this.markTabAsRestoring(tab);
+        } else if (!data.isRemotenessUpdate) {
+          // If the user was typing into the URL bar when we crashed, but hadn't hit
+          // enter yet, then we just need to write that value to the URL bar without
+          // loading anything. This must happen after the load, as the load will clear
+          // userTypedValue.
+          let tabData = TabState.collect(tab);
+          if (tabData.userTypedValue && !tabData.userTypedClear && !browser.userTypedValue) {
+            browser.userTypedValue = tabData.userTypedValue;
+            win.URLBarSetURI();
+          }
+
+          // Remove state we don't need any longer.
+          TabStateCache.update(browser, {
+            userTypedValue: null, userTypedClear: null
+          });
+        }
+        break;
+      case "SessionStore:restoreTabContentComplete":
+        // This callback is used exclusively by tests that want to
+        // monitor the progress of network loads.
+        if (gDebuggingEnabled) {
+          Services.obs.notifyObservers(browser, NOTIFY_TAB_RESTORED, null);
+        }
+
+        SessionStoreInternal._resetLocalTabRestoringState(tab);
+        SessionStoreInternal.restoreNextTab();
+
+        this._sendTabRestoredNotification(tab, data.isRemotenessUpdate);
+        break;
+      case "SessionStore:crashedTabRevived":
+        // The browser was revived by navigating to a different page
+        // manually, so we remove it from the ignored browser set.
+        this._crashedBrowsers.delete(browser.permanentKey);
+        break;
+      case "SessionStore:error":
+        this.reportInternalError(data);
+        TabStateFlusher.resolveAll(browser, false, "Received error from the content process");
+        break;
+      default:
+        throw new Error(`received unknown message '${aMessage.name}'`);
+        break;
+    }
+  },
+
+  /**
+   * Record telemetry measurements stored in an object.
+   * @param telemetry
+   *        {histogramID: value, ...} An object mapping histogramIDs to the
+   *        value to be recorded for that ID,
+   */
+  recordTelemetry: function (telemetry) {
+    for (let histogramId in telemetry){
+      Telemetry.getHistogramById(histogramId).add(telemetry[histogramId]);
+    }
+  },
+
+  /* ........ Window Event Handlers .............. */
+
+  /**
+   * Implement nsIDOMEventListener for handling various window and tab events
+   */
+  handleEvent: function ssi_handleEvent(aEvent) {
+    let win = aEvent.currentTarget.ownerGlobal;
+    let target = aEvent.originalTarget;
+    switch (aEvent.type) {
+      case "TabOpen":
+        this.onTabAdd(win);
+        break;
+      case "TabBrowserInserted":
+        this.onTabBrowserInserted(win, target);
+        break;
+      case "TabClose":
+        // `adoptedBy` will be set if the tab was closed because it is being
+        // moved to a new window.
+        if (!aEvent.detail.adoptedBy)
+          this.onTabClose(win, target);
+        this.onTabRemove(win, target);
+        this._notifyOfClosedObjectsChange();
+        break;
+      case "TabSelect":
+        this.onTabSelect(win);
+        break;
+      case "TabShow":
+        this.onTabShow(win, target);
+        break;
+      case "TabHide":
+        this.onTabHide(win, target);
+        break;
+      case "TabPinned":
+      case "TabUnpinned":
+      case "SwapDocShells":
+        this.saveStateDelayed(win);
+        break;
+      case "oop-browser-crashed":
+        this.onBrowserCrashed(target);
+        break;
+      case "XULFrameLoaderCreated":
+        if (target.namespaceURI == NS_XUL &&
+            target.localName == "browser" &&
+            target.frameLoader &&
+            target.permanentKey) {
+          this._lastKnownFrameLoader.set(target.permanentKey, target.frameLoader);
+          this.resetEpoch(target);
+        }
+        break;
+      case "BrowserWillChangeProcess":
+        let promise = TabStateFlusher.flush(target);
+        target.frameLoader.addProcessChangeBlockingPromise(promise);
+        break;
+      case "BrowserChangedProcess":
+        let newEpoch = 1 + Math.max(this.getCurrentEpoch(target),
+                                    this.getCurrentEpoch(aEvent.otherBrowser));
+        this.setCurrentEpoch(target, newEpoch);
+        target.messageManager.sendAsyncMessage("SessionStore:becomeActiveProcess", {
+          epoch: newEpoch
+        });
+        break;
+      default:
+        throw new Error(`unhandled event ${aEvent.type}?`);
+    }
+    this._clearRestoringWindows();
+  },
+
+  /**
+   * Generate a unique window identifier
+   * @return string
+   *         A unique string to identify a window
+   */
+  _generateWindowID: function ssi_generateWindowID() {
+    return "window" + (this._nextWindowID++);
+  },
+
+  /**
+   * Registers and tracks a given window.
+   *
+   * @param aWindow
+   *        Window reference
+   */
+  onLoad(aWindow) {
+    // return if window has already been initialized
+    if (aWindow && aWindow.__SSi && this._windows[aWindow.__SSi])
+      return;
+
+    // ignore windows opened while shutting down
+    if (RunState.isQuitting)
+      return;
+
+    // Assign the window a unique identifier we can use to reference
+    // internal data about the window.
+    aWindow.__SSi = this._generateWindowID();
+
+    let mm = aWindow.getGroupMessageManager("browsers");
+    MESSAGES.forEach(msg => {
+      let listenWhenClosed = CLOSED_MESSAGES.has(msg);
+      mm.addMessageListener(msg, this, listenWhenClosed);
+    });
+
+    // Load the frame script after registering listeners.
+    mm.loadFrameScript("chrome://browser/content/content-sessionStore.js", true);
+
+    // and create its data object
+    this._windows[aWindow.__SSi] = { tabs: [], selected: 0, _closedTabs: [], busy: false };
+
+    if (PrivateBrowsingUtils.isWindowPrivate(aWindow))
+      this._windows[aWindow.__SSi].isPrivate = true;
+    if (!this._isWindowLoaded(aWindow))
+      this._windows[aWindow.__SSi]._restoring = true;
+    if (!aWindow.toolbar.visible)
+      this._windows[aWindow.__SSi].isPopup = true;
+
+    let tabbrowser = aWindow.gBrowser;
+
+    // add tab change listeners to all already existing tabs
+    for (let i = 0; i < tabbrowser.tabs.length; i++) {
+      this.onTabBrowserInserted(aWindow, tabbrowser.tabs[i]);
+    }
+    // notification of tab add/remove/selection/show/hide
+    TAB_EVENTS.forEach(function(aEvent) {
+      tabbrowser.tabContainer.addEventListener(aEvent, this, true);
+    }, this);
+
+    // Keep track of a browser's latest frameLoader.
+    aWindow.gBrowser.addEventListener("XULFrameLoaderCreated", this);
+    aWindow.gBrowser.addEventListener("BrowserChangedProcess", this);
+    aWindow.gBrowser.addEventListener("BrowserWillChangeProcess", this);
+  },
+
+  /**
+   * Initializes a given window.
+   *
+   * Windows are registered as soon as they are created but we need to wait for
+   * the session file to load, and the initial window's delayed startup to
+   * finish before initializing a window, i.e. restoring data into it.
+   *
+   * @param aWindow
+   *        Window reference
+   * @param aInitialState
+   *        The initial state to be loaded after startup (optional)
+   */
+  initializeWindow(aWindow, aInitialState = null) {
+    let isPrivateWindow = PrivateBrowsingUtils.isWindowPrivate(aWindow);
+
+    // perform additional initialization when the first window is loading
+    if (RunState.isStopped) {
+      RunState.setRunning();
+
+      // restore a crashed session resp. resume the last session if requested
+      if (aInitialState) {
+        // Don't write to disk right after startup. Set the last time we wrote
+        // to disk to NOW() to enforce a full interval before the next write.
+        SessionSaver.updateLastSaveTime();
+
+        if (isPrivateWindow) {
+          // We're starting with a single private window. Save the state we
+          // actually wanted to restore so that we can do it later in case
+          // the user opens another, non-private window.
+          this._deferredInitialState = gSessionStartup.state;
+
+          // Nothing to restore now, notify observers things are complete.
+          Services.obs.notifyObservers(null, NOTIFY_WINDOWS_RESTORED, "");
+        } else {
+          TelemetryTimestamps.add("sessionRestoreRestoring");
+          this._restoreCount = aInitialState.windows ? aInitialState.windows.length : 0;
+
+          // global data must be restored before restoreWindow is called so that
+          // it happens before observers are notified
+          this._globalState.setFromState(aInitialState);
+
+          let overwrite = this._isCmdLineEmpty(aWindow, aInitialState);
+          let options = {firstWindow: true, overwriteTabs: overwrite};
+          this.restoreWindows(aWindow, aInitialState, options);
+        }
+      }
+      else {
+        // Nothing to restore, notify observers things are complete.
+        Services.obs.notifyObservers(null, NOTIFY_WINDOWS_RESTORED, "");
+      }
+    }
+    // this window was opened by _openWindowWithState
+    else if (!this._isWindowLoaded(aWindow)) {
+      let state = this._statesToRestore[aWindow.__SS_restoreID];
+      let options = {overwriteTabs: true, isFollowUp: state.windows.length == 1};
+      this.restoreWindow(aWindow, state.windows[0], options);
+    }
+    // The user opened another, non-private window after starting up with
+    // a single private one. Let's restore the session we actually wanted to
+    // restore at startup.
+    else if (this._deferredInitialState && !isPrivateWindow &&
+             aWindow.toolbar.visible) {
+
+      // global data must be restored before restoreWindow is called so that
+      // it happens before observers are notified
+      this._globalState.setFromState(this._deferredInitialState);
+
+      this._restoreCount = this._deferredInitialState.windows ?
+        this._deferredInitialState.windows.length : 0;
+      this.restoreWindows(aWindow, this._deferredInitialState, {firstWindow: true});
+      this._deferredInitialState = null;
+    }
+    else if (this._restoreLastWindow && aWindow.toolbar.visible &&
+             this._closedWindows.length && !isPrivateWindow) {
+
+      // default to the most-recently closed window
+      // don't use popup windows
+      let closedWindowState = null;
+      let closedWindowIndex;
+      for (let i = 0; i < this._closedWindows.length; i++) {
+        // Take the first non-popup, point our object at it, and break out.
+        if (!this._closedWindows[i].isPopup) {
+          closedWindowState = this._closedWindows[i];
+          closedWindowIndex = i;
+          break;
+        }
+      }
+
+      if (closedWindowState) {
+        let newWindowState;
+#ifdef XP_MACOSX
+        let isMacOS = true;
+#else
+        let isMacOS = false;
+#endif
+        if (isMacOS || !this._doResumeSession()) {
+          // We want to split the window up into pinned tabs and unpinned tabs.
+          // Pinned tabs should be restored. If there are any remaining tabs,
+          // they should be added back to _closedWindows.
+          // We'll cheat a little bit and reuse _prepDataForDeferredRestore
+          // even though it wasn't built exactly for this.
+          let [appTabsState, normalTabsState] =
+            this._prepDataForDeferredRestore({ windows: [closedWindowState] });
+
+          // These are our pinned tabs, which we should restore
+          if (appTabsState.windows.length) {
+            newWindowState = appTabsState.windows[0];
+            delete newWindowState.__lastSessionWindowID;
+          }
+
+          // In case there were no unpinned tabs, remove the window from _closedWindows
+          if (!normalTabsState.windows.length) {
+            this._removeClosedWindow(closedWindowIndex);
+          }
+          // Or update _closedWindows with the modified state
+          else {
+            delete normalTabsState.windows[0].__lastSessionWindowID;
+            this._closedWindows[closedWindowIndex] = normalTabsState.windows[0];
+          }
+        }
+        else {
+          // If we're just restoring the window, make sure it gets removed from
+          // _closedWindows.
+          this._removeClosedWindow(closedWindowIndex);
+          newWindowState = closedWindowState;
+          delete newWindowState.hidden;
+        }
+
+        if (newWindowState) {
+          // Ensure that the window state isn't hidden
+          this._restoreCount = 1;
+          let state = { windows: [newWindowState] };
+          let options = {overwriteTabs: this._isCmdLineEmpty(aWindow, state)};
+          this.restoreWindow(aWindow, newWindowState, options);
+        }
+      }
+      // we actually restored the session just now.
+      this._prefBranch.setBoolPref("sessionstore.resume_session_once", false);
+    }
+    if (this._restoreLastWindow && aWindow.toolbar.visible) {
+      // always reset (if not a popup window)
+      // we don't want to restore a window directly after, for example,
+      // undoCloseWindow was executed.
+      this._restoreLastWindow = false;
+    }
+  },
+
+  /**
+   * Called right before a new browser window is shown.
+   * @param aWindow
+   *        Window reference
+   */
+  onBeforeBrowserWindowShown: function (aWindow) {
+    // Register the window.
+    this.onLoad(aWindow);
+
+    // Just call initializeWindow() directly if we're initialized already.
+    if (this._sessionInitialized) {
+      this.initializeWindow(aWindow);
+      return;
+    }
+
+    // The very first window that is opened creates a promise that is then
+    // re-used by all subsequent windows. The promise will be used to tell
+    // when we're ready for initialization.
+    if (!this._promiseReadyForInitialization) {
+      // Wait for the given window's delayed startup to be finished.
+      let promise = new Promise(resolve => {
+        Services.obs.addObserver(function obs(subject, topic) {
+          if (aWindow == subject) {
+            Services.obs.removeObserver(obs, topic);
+            resolve();
+          }
+        }, "browser-delayed-startup-finished", false);
+      });
+
+      // We are ready for initialization as soon as the session file has been
+      // read from disk and the initial window's delayed startup has finished.
+      this._promiseReadyForInitialization =
+        Promise.all([promise, gSessionStartup.onceInitialized]);
+    }
+
+    // We can't call this.onLoad since initialization
+    // hasn't completed, so we'll wait until it is done.
+    // Even if additional windows are opened and wait
+    // for initialization as well, the first opened
+    // window should execute first, and this.onLoad
+    // will be called with the initialState.
+    this._promiseReadyForInitialization.then(() => {
+      if (aWindow.closed) {
+        return;
+      }
+
+      if (this._sessionInitialized) {
+        this.initializeWindow(aWindow);
+      } else {
+        let initialState = this.initSession();
+        this._sessionInitialized = true;
+
+        if (initialState) {
+          Services.obs.notifyObservers(null, NOTIFY_RESTORING_ON_STARTUP, "");
+        }
+        TelemetryStopwatch.start("FX_SESSION_RESTORE_STARTUP_ONLOAD_INITIAL_WINDOW_MS");
+        this.initializeWindow(aWindow, initialState);
+        TelemetryStopwatch.finish("FX_SESSION_RESTORE_STARTUP_ONLOAD_INITIAL_WINDOW_MS");
+
+        // Let everyone know we're done.
+        this._deferredInitialized.resolve();
+      }
+    }, console.error);
+  },
+
+  /**
+   * On window close...
+   * - remove event listeners from tabs
+   * - save all window data
+   * @param aWindow
+   *        Window reference
+   *
+   * @returns a Promise
+   */
+  onClose: function ssi_onClose(aWindow) {
+    let completionPromise = Promise.resolve();
+    // this window was about to be restored - conserve its original data, if any
+    let isFullyLoaded = this._isWindowLoaded(aWindow);
+    if (!isFullyLoaded) {
+      if (!aWindow.__SSi) {
+        aWindow.__SSi = this._generateWindowID();
+      }
+
+      this._windows[aWindow.__SSi] = this._statesToRestore[aWindow.__SS_restoreID];
+      delete this._statesToRestore[aWindow.__SS_restoreID];
+      delete aWindow.__SS_restoreID;
+    }
+
+    // ignore windows not tracked by SessionStore
+    if (!aWindow.__SSi || !this._windows[aWindow.__SSi]) {
+      return completionPromise;
+    }
+
+    // notify that the session store will stop tracking this window so that
+    // extensions can store any data about this window in session store before
+    // that's not possible anymore
+    let event = aWindow.document.createEvent("Events");
+    event.initEvent("SSWindowClosing", true, false);
+    aWindow.dispatchEvent(event);
+
+    if (this.windowToFocus && this.windowToFocus == aWindow) {
+      delete this.windowToFocus;
+    }
+
+    var tabbrowser = aWindow.gBrowser;
+
+    let browsers = Array.from(tabbrowser.browsers);
+
+    TAB_EVENTS.forEach(function(aEvent) {
+      tabbrowser.tabContainer.removeEventListener(aEvent, this, true);
+    }, this);
+
+    aWindow.gBrowser.removeEventListener("XULFrameLoaderCreated", this);
+    aWindow.gBrowser.removeEventListener("BrowserChangedProcess", this);
+    aWindow.gBrowser.removeEventListener("BrowserWillChangeProcess", this);
+
+    let winData = this._windows[aWindow.__SSi];
+
+    // Collect window data only when *not* closed during shutdown.
+    if (RunState.isRunning) {
+      // Grab the most recent window data. The tab data will be updated
+      // once we finish flushing all of the messages from the tabs.
+      let tabMap = this._collectWindowData(aWindow);
+
+      for (let [tab, tabData] of tabMap) {
+        let permanentKey = tab.linkedBrowser.permanentKey;
+        this._closedWindowTabs.set(permanentKey, tabData);
+      }
+
+      if (isFullyLoaded) {
+        winData.title = tabbrowser.selectedBrowser.contentTitle || tabbrowser.selectedTab.label;
+        winData.title = this._replaceLoadingTitle(winData.title, tabbrowser,
+                                                  tabbrowser.selectedTab);
+        SessionCookies.update([winData]);
+      }
+
+#ifndef XP_MACOSX
+      // Until we decide otherwise elsewhere, this window is part of a series
+      // of closing windows to quit.
+      winData._shouldRestore = true;
+#endif
+
+      // Store the window's close date to figure out when each individual tab
+      // was closed. This timestamp should allow re-arranging data based on how
+      // recently something was closed.
+      winData.closedAt = Date.now();
+
+      // we don't want to save the busy state
+      delete winData.busy;
+
+      // When closing windows one after the other until Firefox quits, we
+      // will move those closed in series back to the "open windows" bucket
+      // before writing to disk. If however there is only a single window
+      // with tabs we deem not worth saving then we might end up with a
+      // random closed or even a pop-up window re-opened. To prevent that
+      // we explicitly allow saving an "empty" window state.
+      let isLastWindow =
+        Object.keys(this._windows).length == 1 &&
+        !this._closedWindows.some(win => win._shouldRestore || false);
+
+      // clear this window from the list, since it has definitely been closed.
+      delete this._windows[aWindow.__SSi];
+
+      // This window has the potential to be saved in the _closedWindows
+      // array (maybeSaveClosedWindows gets the final call on that).
+      this._saveableClosedWindowData.add(winData);
+
+      // Now we have to figure out if this window is worth saving in the _closedWindows
+      // Object.
+      //
+      // We're about to flush the tabs from this window, but it's possible that we
+      // might never hear back from the content process(es) in time before the user
+      // chooses to restore the closed window. So we do the following:
+      //
+      // 1) Use the tab state cache to determine synchronously if the window is
+      //    worth stashing in _closedWindows.
+      // 2) Flush the window.
+      // 3) When the flush is complete, revisit our decision to store the window
+      //    in _closedWindows, and add/remove as necessary.
+      if (!winData.isPrivate) {
+        // Remove any open private tabs the window may contain.
+        PrivacyFilter.filterPrivateTabs(winData);
+        this.maybeSaveClosedWindow(winData, isLastWindow);
+      }
+
+      completionPromise = TabStateFlusher.flushWindow(aWindow).then(() => {
+        // At this point, aWindow is closed! You should probably not try to
+        // access any DOM elements from aWindow within this callback unless
+        // you're holding on to them in the closure.
+
+        for (let browser of browsers) {
+          if (this._closedWindowTabs.has(browser.permanentKey)) {
+            let tabData = this._closedWindowTabs.get(browser.permanentKey);
+            TabState.copyFromCache(browser, tabData);
+            this._closedWindowTabs.delete(browser.permanentKey);
+          }
+        }
+
+        // Save non-private windows if they have at
+        // least one saveable tab or are the last window.
+        if (!winData.isPrivate) {
+          // It's possible that a tab switched its privacy state at some point
+          // before our flush, so we need to filter again.
+          PrivacyFilter.filterPrivateTabs(winData);
+          this.maybeSaveClosedWindow(winData, isLastWindow);
+        }
+
+        // Update the tabs data now that we've got the most
+        // recent information.
+        this.cleanUpWindow(aWindow, winData, browsers);
+
+        // save the state without this window to disk
+        this.saveStateDelayed();
+      });
+    } else {
+      this.cleanUpWindow(aWindow, winData, browsers);
+    }
+
+    for (let i = 0; i < tabbrowser.tabs.length; i++) {
+      this.onTabRemove(aWindow, tabbrowser.tabs[i], true);
+    }
+
+    return completionPromise;
+  },
+
+  /**
+   * Clean up the message listeners on a window that has finally
+   * gone away. Call this once you're sure you don't want to hear
+   * from any of this windows tabs from here forward.
+   *
+   * @param aWindow
+   *        The browser window we're cleaning up.
+   * @param winData
+   *        The data for the window that we should hold in the
+   *        DyingWindowCache in case anybody is still holding a
+   *        reference to it.
+   */
+  cleanUpWindow(aWindow, winData, browsers) {
+    // Any leftover TabStateFlusher Promises need to be resolved now,
+    // since we're about to remove the message listeners.
+    for (let browser of browsers) {
+      TabStateFlusher.resolveAll(browser);
+    }
+
+    // Cache the window state until it is completely gone.
+    DyingWindowCache.set(aWindow, winData);
+
+    let mm = aWindow.getGroupMessageManager("browsers");
+    MESSAGES.forEach(msg => mm.removeMessageListener(msg, this));
+
+    this._saveableClosedWindowData.delete(winData);
+    delete aWindow.__SSi;
+  },
+
+  /**
+   * Decides whether or not a closed window should be put into the
+   * _closedWindows Object. This might be called multiple times per
+   * window, and will do the right thing of moving the window data
+   * in or out of _closedWindows if the winData indicates that our
+   * need for saving it has changed.
+   *
+   * @param winData
+   *        The data for the closed window that we might save.
+   * @param isLastWindow
+   *        Whether or not the window being closed is the last
+   *        browser window. Callers of this function should pass
+   *        in the value of SessionStoreInternal.atLastWindow for
+   *        this argument, and pass in the same value if they happen
+   *        to call this method again asynchronously (for example, after
+   *        a window flush).
+   */
+  maybeSaveClosedWindow(winData, isLastWindow) {
+    // Make sure SessionStore is still running, and make sure that we
+    // haven't chosen to forget this window.
+    if (RunState.isRunning && this._saveableClosedWindowData.has(winData)) {
+      // Determine whether the window has any tabs worth saving.
+      let hasSaveableTabs = winData.tabs.some(this._shouldSaveTabState);
+
+      // Note that we might already have this window stored in
+      // _closedWindows from a previous call to this function.
+      let winIndex = this._closedWindows.indexOf(winData);
+      let alreadyStored = (winIndex != -1);
+      let shouldStore = (hasSaveableTabs || isLastWindow);
+
+      if (shouldStore && !alreadyStored) {
+        let index = this._closedWindows.findIndex(win => {
+          return win.closedAt < winData.closedAt;
+        });
+
+        // If we found no tab closed before our
+        // tab then just append it to the list.
+        if (index == -1) {
+          index = this._closedWindows.length;
+        }
+
+        // About to save the closed window, add a unique ID.
+        winData.closedId = this._nextClosedId++;
+
+        // Insert tabData at the right position.
+        this._closedWindows.splice(index, 0, winData);
+        this._capClosedWindows();
+        this._closedObjectsChanged = true;
+      } else if (!shouldStore && alreadyStored) {
+        this._removeClosedWindow(winIndex);
+      }
+    }
+  },
+
+  /**
+   * On quit application granted
+   */
+  onQuitApplicationGranted: function ssi_onQuitApplicationGranted(syncShutdown=false) {
+    // Collect an initial snapshot of window data before we do the flush
+    this._forEachBrowserWindow((win) => {
+      this._collectWindowData(win);
+    });
+
+    // Now add an AsyncShutdown blocker that'll spin the event loop
+    // until the windows have all been flushed.
+
+    // This progress object will track the state of async window flushing
+    // and will help us debug things that go wrong with our AsyncShutdown
+    // blocker.
+    let progress = { total: -1, current: -1 };
+
+    // We're going down! Switch state so that we treat closing windows and
+    // tabs correctly.
+    RunState.setQuitting();
+
+    if (!syncShutdown) {
+      // We've got some time to shut down, so let's do this properly.
+      // To prevent blocker from breaking the 60 sec limit(which will cause a
+      // crash) of async shutdown during flushing all windows, we resolve the
+      // promise passed to blocker once:
+      // 1. the flushing exceed 50 sec, or
+      // 2. 'oop-frameloader-crashed' or 'ipc:content-shutdown' is observed.
+      // Thus, Firefox still can open the last session on next startup.
+      AsyncShutdown.quitApplicationGranted.addBlocker(
+        "SessionStore: flushing all windows",
+        () => {
+          var promises = [];
+          promises.push(this.flushAllWindowsAsync(progress));
+          promises.push(this.looseTimer(50000));
+
+          var promiseOFC = new Promise(resolve => {
+            Services.obs.addObserver(function obs(subject, topic) {
+              Services.obs.removeObserver(obs, topic);
+              resolve();
+            }, "oop-frameloader-crashed", false);
+          });
+          promises.push(promiseOFC);
+
+          var promiseICS = new Promise(resolve => {
+            Services.obs.addObserver(function obs(subject, topic) {
+              Services.obs.removeObserver(obs, topic);
+              resolve();
+            }, "ipc:content-shutdown", false);
+          });
+          promises.push(promiseICS);
+
+          return Promise.race(promises);
+        },
+        () => progress);
+    } else {
+      // We have to shut down NOW, which means we only get to save whatever
+      // we already had cached.
+    }
+  },
+
+  /**
+   * An async Task that iterates all open browser windows and flushes
+   * any outstanding messages from their tabs. This will also close
+   * all of the currently open windows while we wait for the flushes
+   * to complete.
+   *
+   * @param progress (Object)
+   *        Optional progress object that will be updated as async
+   *        window flushing progresses. flushAllWindowsSync will
+   *        write to the following properties:
+   *
+   *        total (int):
+   *          The total number of windows to be flushed.
+   *        current (int):
+   *          The current window that we're waiting for a flush on.
+   *
+   * @return Promise
+   */
+  flushAllWindowsAsync: Task.async(function*(progress={}) {
+    let windowPromises = new Map();
+    // We collect flush promises and close each window immediately so that
+    // the user can't start changing any window state while we're waiting
+    // for the flushes to finish.
+    this._forEachBrowserWindow((win) => {
+      windowPromises.set(win, TabStateFlusher.flushWindow(win));
+
+      // We have to wait for these messages to come up from
+      // each window and each browser. In the meantime, hide
+      // the windows to improve perceived shutdown speed.
+      let baseWin = win.QueryInterface(Ci.nsIInterfaceRequestor)
+                       .getInterface(Ci.nsIDocShell)
+                       .QueryInterface(Ci.nsIDocShellTreeItem)
+                       .treeOwner
+                       .QueryInterface(Ci.nsIBaseWindow);
+      baseWin.visibility = false;
+    });
+
+    progress.total = windowPromises.size;
+    progress.current = 0;
+
+    // We'll iterate through the Promise array, yielding each one, so as to
+    // provide useful progress information to AsyncShutdown.
+    for (let [win, promise] of windowPromises) {
+      yield promise;
+      this._collectWindowData(win);
+      progress.current++;
+    };
+
+    // We must cache this because _getMostRecentBrowserWindow will always
+    // return null by the time quit-application occurs.
+    var activeWindow = this._getMostRecentBrowserWindow();
+    if (activeWindow)
+      this.activeWindowSSiCache = activeWindow.__SSi || "";
+    DirtyWindows.clear();
+  }),
+
+  /**
+   * On last browser window close
+   */
+  onLastWindowCloseGranted: function ssi_onLastWindowCloseGranted() {
+    // last browser window is quitting.
+    // remember to restore the last window when another browser window is opened
+    // do not account for pref(resume_session_once) at this point, as it might be
+    // set by another observer getting this notice after us
+    this._restoreLastWindow = true;
+  },
+
+  /**
+   * On quitting application
+   * @param aData
+   *        String type of quitting
+   */
+  onQuitApplication: function ssi_onQuitApplication(aData) {
+    if (aData == "restart") {
+      this._prefBranch.setBoolPref("sessionstore.resume_session_once", true);
+      // The browser:purge-session-history notification fires after the
+      // quit-application notification so unregister the
+      // browser:purge-session-history notification to prevent clearing
+      // session data on disk on a restart.  It is also unnecessary to
+      // perform any other sanitization processing on a restart as the
+      // browser is about to exit anyway.
+      Services.obs.removeObserver(this, "browser:purge-session-history");
+    }
+
+    if (aData != "restart") {
+      // Throw away the previous session on shutdown
+      LastSession.clear();
+    }
+
+    this._uninit();
+  },
+
+  /**
+   * On purge of session history
+   */
+  onPurgeSessionHistory: function ssi_onPurgeSessionHistory() {
+    SessionFile.wipe();
+    // If the browser is shutting down, simply return after clearing the
+    // session data on disk as this notification fires after the
+    // quit-application notification so the browser is about to exit.
+    if (RunState.isQuitting)
+      return;
+    LastSession.clear();
+
+    let openWindows = {};
+    // Collect open windows.
+    this._forEachBrowserWindow(({__SSi: id}) => openWindows[id] = true);
+
+    // also clear all data about closed tabs and windows
+    for (let ix in this._windows) {
+      if (ix in openWindows) {
+        if (this._windows[ix]._closedTabs.length) {
+          this._windows[ix]._closedTabs = [];
+          this._closedObjectsChanged = true;
+        }
+      } else {
+        delete this._windows[ix];
+      }
+    }
+    // also clear all data about closed windows
+    if (this._closedWindows.length) {
+      this._closedWindows = [];
+      this._closedObjectsChanged = true;
+    }
+    // give the tabbrowsers a chance to clear their histories first
+    var win = this._getMostRecentBrowserWindow();
+    if (win) {
+      win.setTimeout(() => SessionSaver.run(), 0);
+    } else if (RunState.isRunning) {
+      SessionSaver.run();
+    }
+
+    this._clearRestoringWindows();
+    this._saveableClosedWindowData = new WeakSet();
+  },
+
+  /**
+   * On purge of domain data
+   * @param aData
+   *        String domain data
+   */
+  onPurgeDomainData: function ssi_onPurgeDomainData(aData) {
+    // does a session history entry contain a url for the given domain?
+    function containsDomain(aEntry) {
+      if (Utils.hasRootDomain(aEntry.url, aData)) {
+        return true;
+      }
+      return aEntry.children && aEntry.children.some(containsDomain, this);
+    }
+    // remove all closed tabs containing a reference to the given domain
+    for (let ix in this._windows) {
+      let closedTabs = this._windows[ix]._closedTabs;
+      for (let i = closedTabs.length - 1; i >= 0; i--) {
+        if (closedTabs[i].state.entries.some(containsDomain, this)) {
+          closedTabs.splice(i, 1);
+          this._closedObjectsChanged = true;
+        }
+      }
+    }
+    // remove all open & closed tabs containing a reference to the given
+    // domain in closed windows
+    for (let ix = this._closedWindows.length - 1; ix >= 0; ix--) {
+      let closedTabs = this._closedWindows[ix]._closedTabs;
+      let openTabs = this._closedWindows[ix].tabs;
+      let openTabCount = openTabs.length;
+      for (let i = closedTabs.length - 1; i >= 0; i--)
+        if (closedTabs[i].state.entries.some(containsDomain, this))
+          closedTabs.splice(i, 1);
+      for (let j = openTabs.length - 1; j >= 0; j--) {
+        if (openTabs[j].entries.some(containsDomain, this)) {
+          openTabs.splice(j, 1);
+          if (this._closedWindows[ix].selected > j)
+            this._closedWindows[ix].selected--;
+        }
+      }
+      if (openTabs.length == 0) {
+        this._closedWindows.splice(ix, 1);
+      }
+      else if (openTabs.length != openTabCount) {
+        // Adjust the window's title if we removed an open tab
+        let selectedTab = openTabs[this._closedWindows[ix].selected - 1];
+        // some duplication from restoreHistory - make sure we get the correct title
+        let activeIndex = (selectedTab.index || selectedTab.entries.length) - 1;
+        if (activeIndex >= selectedTab.entries.length)
+          activeIndex = selectedTab.entries.length - 1;
+        this._closedWindows[ix].title = selectedTab.entries[activeIndex].title;
+      }
+    }
+
+    if (RunState.isRunning) {
+      SessionSaver.run();
+    }
+
+    this._clearRestoringWindows();
+  },
+
+  /**
+   * On preference change
+   * @param aData
+   *        String preference changed
+   */
+  onPrefChange: function ssi_onPrefChange(aData) {
+    switch (aData) {
+      // if the user decreases the max number of closed tabs they want
+      // preserved update our internal states to match that max
+      case "sessionstore.max_tabs_undo":
+        this._max_tabs_undo = this._prefBranch.getIntPref("sessionstore.max_tabs_undo");
+        for (let ix in this._windows) {
+          if (this._windows[ix]._closedTabs.length > this._max_tabs_undo) {
+            this._windows[ix]._closedTabs.splice(this._max_tabs_undo, this._windows[ix]._closedTabs.length);
+            this._closedObjectsChanged = true;
+          }
+        }
+        break;
+      case "sessionstore.max_windows_undo":
+        this._max_windows_undo = this._prefBranch.getIntPref("sessionstore.max_windows_undo");
+        this._capClosedWindows();
+        break;
+    }
+  },
+
+  /**
+   * save state when new tab is added
+   * @param aWindow
+   *        Window reference
+   */
+  onTabAdd: function ssi_onTabAdd(aWindow) {
+    this.saveStateDelayed(aWindow);
+  },
+
+  /**
+   * set up listeners for a new tab
+   * @param aWindow
+   *        Window reference
+   * @param aTab
+   *        Tab reference
+   */
+  onTabBrowserInserted: function ssi_onTabBrowserInserted(aWindow, aTab) {
+    let browser = aTab.linkedBrowser;
+    browser.addEventListener("SwapDocShells", this);
+    browser.addEventListener("oop-browser-crashed", this);
+
+    if (browser.frameLoader) {
+      this._lastKnownFrameLoader.set(browser.permanentKey, browser.frameLoader);
+    }
+  },
+
+  /**
+   * remove listeners for a tab
+   * @param aWindow
+   *        Window reference
+   * @param aTab
+   *        Tab reference
+   * @param aNoNotification
+   *        bool Do not save state if we're updating an existing tab
+   */
+  onTabRemove: function ssi_onTabRemove(aWindow, aTab, aNoNotification) {
+    let browser = aTab.linkedBrowser;
+    browser.removeEventListener("SwapDocShells", this);
+    browser.removeEventListener("oop-browser-crashed", this);
+
+    // If this tab was in the middle of restoring or still needs to be restored,
+    // we need to reset that state. If the tab was restoring, we will attempt to
+    // restore the next tab.
+    let previousState = browser.__SS_restoreState;
+    if (previousState) {
+      this._resetTabRestoringState(aTab);
+      if (previousState == TAB_STATE_RESTORING)
+        this.restoreNextTab();
+    }
+
+    if (!aNoNotification) {
+      this.saveStateDelayed(aWindow);
+    }
+  },
+
+  /**
+   * When a tab closes, collect its properties
+   * @param aWindow
+   *        Window reference
+   * @param aTab
+   *        Tab reference
+   */
+  onTabClose: function ssi_onTabClose(aWindow, aTab) {
+    // notify the tabbrowser that the tab state will be retrieved for the last time
+    // (so that extension authors can easily set data on soon-to-be-closed tabs)
+    var event = aWindow.document.createEvent("Events");
+    event.initEvent("SSTabClosing", true, false);
+    aTab.dispatchEvent(event);
+
+    // don't update our internal state if we don't have to
+    if (this._max_tabs_undo == 0) {
+      return;
+    }
+
+    // Get the latest data for this tab (generally, from the cache)
+    let tabState = TabState.collect(aTab);
+
+    // Don't save private tabs
+    let isPrivateWindow = PrivateBrowsingUtils.isWindowPrivate(aWindow);
+    if (!isPrivateWindow && tabState.isPrivate) {
+      return;
+    }
+
+    // Store closed-tab data for undo.
+    let tabbrowser = aWindow.gBrowser;
+    let tabTitle = this._replaceLoadingTitle(aTab.label, tabbrowser, aTab);
+    let {permanentKey} = aTab.linkedBrowser;
+
+    let tabData = {
+      permanentKey,
+      state: tabState,
+      title: tabTitle,
+      image: tabbrowser.getIcon(aTab),
+      iconLoadingPrincipal: Utils.serializePrincipal(aTab.linkedBrowser.contentPrincipal),
+      pos: aTab._tPos,
+      closedAt: Date.now()
+    };
+
+    let closedTabs = this._windows[aWindow.__SSi]._closedTabs;
+
+    // Determine whether the tab contains any information worth saving. Note
+    // that there might be pending state changes queued in the child that
+    // didn't reach the parent yet. If a tab is emptied before closing then we
+    // might still remove it from the list of closed tabs later.
+    if (this._shouldSaveTabState(tabState)) {
+      // Save the tab state, for now. We might push a valid tab out
+      // of the list but those cases should be extremely rare and
+      // do probably never occur when using the browser normally.
+      // (Tests or add-ons might do weird things though.)
+      this.saveClosedTabData(closedTabs, tabData);
+    }
+
+    // Remember the closed tab to properly handle any last updates included in
+    // the final "update" message sent by the frame script's unload handler.
+    this._closedTabs.set(permanentKey, {closedTabs, tabData});
+  },
+
+  /**
+   * Insert a given |tabData| object into the list of |closedTabs|. We will
+   * determine the right insertion point based on the .closedAt properties of
+   * all tabs already in the list. The list will be truncated to contain a
+   * maximum of |this._max_tabs_undo| entries.
+   *
+   * @param closedTabs (array)
+   *        The list of closed tabs for a window.
+   * @param tabData (object)
+   *        The tabData to be inserted.
+   */
+  saveClosedTabData(closedTabs, tabData) {
+    // Find the index of the first tab in the list
+    // of closed tabs that was closed before our tab.
+    let index = closedTabs.findIndex(tab => {
+      return tab.closedAt < tabData.closedAt;
+    });
+
+    // If we found no tab closed before our
+    // tab then just append it to the list.
+    if (index == -1) {
+      index = closedTabs.length;
+    }
+
+    // About to save the closed tab, add a unique ID.
+    tabData.closedId = this._nextClosedId++;
+
+    // Insert tabData at the right position.
+    closedTabs.splice(index, 0, tabData);
+    this._closedObjectsChanged = true;
+
+    // Truncate the list of closed tabs, if needed.
+    if (closedTabs.length > this._max_tabs_undo) {
+      closedTabs.splice(this._max_tabs_undo, closedTabs.length);
+    }
+  },
+
+  /**
+   * Remove the closed tab data at |index| from the list of |closedTabs|. If
+   * the tab's final message is still pending we will simply discard it when
+   * it arrives so that the tab doesn't reappear in the list.
+   *
+   * @param closedTabs (array)
+   *        The list of closed tabs for a window.
+   * @param index (uint)
+   *        The index of the tab to remove.
+   */
+  removeClosedTabData(closedTabs, index) {
+    // Remove the given index from the list.
+    let [closedTab] = closedTabs.splice(index, 1);
+    this._closedObjectsChanged = true;
+
+    // If the closed tab's state still has a .permanentKey property then we
+    // haven't seen its final update message yet. Remove it from the map of
+    // closed tabs so that we will simply discard its last messages and will
+    // not add it back to the list of closed tabs again.
+    if (closedTab.permanentKey) {
+      this._closedTabs.delete(closedTab.permanentKey);
+      this._closedWindowTabs.delete(closedTab.permanentKey);
+      delete closedTab.permanentKey;
+    }
+
+    return closedTab;
+  },
+
+  /**
+   * When a tab is selected, save session data
+   * @param aWindow
+   *        Window reference
+   */
+  onTabSelect: function ssi_onTabSelect(aWindow) {
+    if (RunState.isRunning) {
+      this._windows[aWindow.__SSi].selected = aWindow.gBrowser.tabContainer.selectedIndex;
+
+      let tab = aWindow.gBrowser.selectedTab;
+      let browser = tab.linkedBrowser;
+
+      if (browser.__SS_restoreState &&
+          browser.__SS_restoreState == TAB_STATE_NEEDS_RESTORE) {
+        // If __SS_restoreState is still on the browser and it is
+        // TAB_STATE_NEEDS_RESTORE, then then we haven't restored
+        // this tab yet.
+        //
+        // It's possible that this tab was recently revived, and that
+        // we've deferred showing the tab crashed page for it (if the
+        // tab crashed in the background). If so, we need to re-enter
+        // the crashed state, since we'll be showing the tab crashed
+        // page.
+        if (TabCrashHandler.willShowCrashedTab(browser)) {
+          this.enterCrashedState(browser);
+        } else {
+          this.restoreTabContent(tab);
+        }
+      }
+    }
+  },
+
+  onTabShow: function ssi_onTabShow(aWindow, aTab) {
+    // If the tab hasn't been restored yet, move it into the right bucket
+    if (aTab.linkedBrowser.__SS_restoreState &&
+        aTab.linkedBrowser.__SS_restoreState == TAB_STATE_NEEDS_RESTORE) {
+      TabRestoreQueue.hiddenToVisible(aTab);
+
+      // let's kick off tab restoration again to ensure this tab gets restored
+      // with "restore_hidden_tabs" == false (now that it has become visible)
+      this.restoreNextTab();
+    }
+
+    // Default delay of 2 seconds gives enough time to catch multiple TabShow
+    // events. This used to be due to changing groups in 'tab groups'. We
+    // might be able to get rid of this now?
+    this.saveStateDelayed(aWindow);
+  },
+
+  onTabHide: function ssi_onTabHide(aWindow, aTab) {
+    // If the tab hasn't been restored yet, move it into the right bucket
+    if (aTab.linkedBrowser.__SS_restoreState &&
+        aTab.linkedBrowser.__SS_restoreState == TAB_STATE_NEEDS_RESTORE) {
+      TabRestoreQueue.visibleToHidden(aTab);
+    }
+
+    // Default delay of 2 seconds gives enough time to catch multiple TabHide
+    // events. This used to be due to changing groups in 'tab groups'. We
+    // might be able to get rid of this now?
+    this.saveStateDelayed(aWindow);
+  },
+
+  /**
+   * Handler for the event that is fired when a <xul:browser> crashes.
+   *
+   * @param aWindow
+   *        The window that the crashed browser belongs to.
+   * @param aBrowser
+   *        The <xul:browser> that is now in the crashed state.
+   */
+  onBrowserCrashed: function(aBrowser) {
+    NS_ASSERT(aBrowser.isRemoteBrowser,
+              "Only remote browsers should be able to crash");
+
+    this.enterCrashedState(aBrowser);
+    // The browser crashed so we might never receive flush responses.
+    // Resolve all pending flush requests for the crashed browser.
+    TabStateFlusher.resolveAll(aBrowser);
+  },
+
+  /**
+   * Called when a browser is showing or is about to show the tab
+   * crashed page. This method causes SessionStore to ignore the
+   * tab until it's restored.
+   *
+   * @param browser
+   *        The <xul:browser> that is about to show the crashed page.
+   */
+  enterCrashedState(browser) {
+    this._crashedBrowsers.add(browser.permanentKey);
+
+    let win = browser.ownerGlobal;
+
+    // If we hadn't yet restored, or were still in the midst of
+    // restoring this browser at the time of the crash, we need
+    // to reset its state so that we can try to restore it again
+    // when the user revives the tab from the crash.
+    if (browser.__SS_restoreState) {
+      let tab = win.gBrowser.getTabForBrowser(browser);
+      this._resetLocalTabRestoringState(tab);
+    }
+  },
+
+  // Clean up data that has been closed a long time ago.
+  // Do not reschedule a save. This will wait for the next regular
+  // save.
+  onIdleDaily: function() {
+    // Remove old closed windows
+    this._cleanupOldData([this._closedWindows]);
+
+    // Remove closed tabs of closed windows
+    this._cleanupOldData(this._closedWindows.map((winData) => winData._closedTabs));
+
+    // Remove closed tabs of open windows
+    this._cleanupOldData(Object.keys(this._windows).map((key) => this._windows[key]._closedTabs));
+
+    this._notifyOfClosedObjectsChange();
+  },
+
+  // Remove "old" data from an array
+  _cleanupOldData: function(targets) {
+    const TIME_TO_LIVE = this._prefBranch.getIntPref("sessionstore.cleanup.forget_closed_after");
+    const now = Date.now();
+
+    for (let array of targets) {
+      for (let i = array.length - 1; i >= 0; --i)  {
+        let data = array[i];
+        // Make sure that we have a timestamp to tell us when the target
+        // has been closed. If we don't have a timestamp, default to a
+        // safe timestamp: just now.
+        data.closedAt = data.closedAt || now;
+        if (now - data.closedAt > TIME_TO_LIVE) {
+          array.splice(i, 1);
+          this._closedObjectsChanged = true;
+        }
+      }
+    }
+  },
+
+  /* ........ nsISessionStore API .............. */
+
+  getBrowserState: function ssi_getBrowserState() {
+    let state = this.getCurrentState();
+
+    // Don't include the last session state in getBrowserState().
+    delete state.lastSessionState;
+
+    // Don't include any deferred initial state.
+    delete state.deferredInitialState;
+
+    return JSON.stringify(state);
+  },
+
+  setBrowserState: function ssi_setBrowserState(aState) {
+    this._handleClosedWindows();
+
+    try {
+      var state = JSON.parse(aState);
+    }
+    catch (ex) { /* invalid state object - don't restore anything */ }
+    if (!state) {
+      throw Components.Exception("Invalid state string: not JSON", Cr.NS_ERROR_INVALID_ARG);
+    }
+    if (!state.windows) {
+      throw Components.Exception("No windows", Cr.NS_ERROR_INVALID_ARG);
+    }
+
+    this._browserSetState = true;
+
+    // Make sure the priority queue is emptied out
+    this._resetRestoringState();
+
+    var window = this._getMostRecentBrowserWindow();
+    if (!window) {
+      this._restoreCount = 1;
+      this._openWindowWithState(state);
+      return;
+    }
+
+    // close all other browser windows
+    this._forEachBrowserWindow(function(aWindow) {
+      if (aWindow != window) {
+        aWindow.close();
+        this.onClose(aWindow);
+      }
+    });
+
+    // make sure closed window data isn't kept
+    if (this._closedWindows.length) {
+      this._closedWindows = [];
+      this._closedObjectsChanged = true;
+    }
+
+    // determine how many windows are meant to be restored
+    this._restoreCount = state.windows ? state.windows.length : 0;
+
+    // global data must be restored before restoreWindow is called so that
+    // it happens before observers are notified
+    this._globalState.setFromState(state);
+
+    // restore to the given state
+    this.restoreWindows(window, state, {overwriteTabs: true});
+
+    // Notify of changes to closed objects.
+    this._notifyOfClosedObjectsChange();
+  },
+
+  getWindowState: function ssi_getWindowState(aWindow) {
+    if ("__SSi" in aWindow) {
+      return JSON.stringify(this._getWindowState(aWindow));
+    }
+
+    if (DyingWindowCache.has(aWindow)) {
+      let data = DyingWindowCache.get(aWindow);
+      return JSON.stringify({ windows: [data] });
+    }
+
+    throw Components.Exception("Window is not tracked", Cr.NS_ERROR_INVALID_ARG);
+  },
+
+  setWindowState: function ssi_setWindowState(aWindow, aState, aOverwrite) {
+    if (!aWindow.__SSi) {
+      throw Components.Exception("Window is not tracked", Cr.NS_ERROR_INVALID_ARG);
+    }
+
+    this.restoreWindows(aWindow, aState, {overwriteTabs: aOverwrite});
+
+    // Notify of changes to closed objects.
+    this._notifyOfClosedObjectsChange();
+  },
+
+  getTabState: function ssi_getTabState(aTab) {
+    if (!aTab.ownerGlobal.__SSi) {
+      throw Components.Exception("Default view is not tracked", Cr.NS_ERROR_INVALID_ARG);
+    }
+
+    let tabState = TabState.collect(aTab);
+
+    return JSON.stringify(tabState);
+  },
+
+  setTabState(aTab, aState) {
+    // Remove the tab state from the cache.
+    // Note that we cannot simply replace the contents of the cache
+    // as |aState| can be an incomplete state that will be completed
+    // by |restoreTabs|.
+    let tabState = JSON.parse(aState);
+    if (!tabState) {
+      throw Components.Exception("Invalid state string: not JSON", Cr.NS_ERROR_INVALID_ARG);
+    }
+    if (typeof tabState != "object") {
+      throw Components.Exception("Not an object", Cr.NS_ERROR_INVALID_ARG);
+    }
+    if (!("entries" in tabState)) {
+      throw Components.Exception("Invalid state object: no entries", Cr.NS_ERROR_INVALID_ARG);
+    }
+
+    let window = aTab.ownerGlobal;
+    if (!("__SSi" in window)) {
+      throw Components.Exception("Window is not tracked", Cr.NS_ERROR_INVALID_ARG);
+    }
+
+    if (aTab.linkedBrowser.__SS_restoreState) {
+      this._resetTabRestoringState(aTab);
+    }
+
+    this.restoreTab(aTab, tabState);
+
+    // Notify of changes to closed objects.
+    this._notifyOfClosedObjectsChange();
+  },
+
+  duplicateTab: function ssi_duplicateTab(aWindow, aTab, aDelta = 0, aRestoreImmediately = true) {
+    if (!aTab.ownerGlobal.__SSi) {
+      throw Components.Exception("Default view is not tracked", Cr.NS_ERROR_INVALID_ARG);
+    }
+    if (!aWindow.gBrowser) {
+      throw Components.Exception("Invalid window object: no gBrowser", Cr.NS_ERROR_INVALID_ARG);
+    }
+
+    // Create a new tab.
+    let userContextId = aTab.getAttribute("usercontextid");
+    let newTab = aTab == aWindow.gBrowser.selectedTab ?
+      aWindow.gBrowser.addTab(null, {relatedToCurrent: true, ownerTab: aTab, userContextId}) :
+      aWindow.gBrowser.addTab(null, {userContextId});
+
+    // Set tab title to "Connecting..." and start the throbber to pretend we're
+    // doing something while actually waiting for data from the frame script.
+    aWindow.gBrowser.setTabTitleLoading(newTab);
+    newTab.setAttribute("busy", "true");
+
+    // Collect state before flushing.
+    let tabState = TabState.clone(aTab);
+
+    // Flush to get the latest tab state to duplicate.
+    let browser = aTab.linkedBrowser;
+    TabStateFlusher.flush(browser).then(() => {
+      // The new tab might have been closed in the meantime.
+      if (newTab.closing || !newTab.linkedBrowser) {
+        return;
+      }
+
+      let window = newTab.ownerGlobal;
+
+      // The tab or its window might be gone.
+      if (!window || !window.__SSi) {
+        return;
+      }
+
+      // Update state with flushed data. We can't use TabState.clone() here as
+      // the tab to duplicate may have already been closed. In that case we
+      // only have access to the <xul:browser>.
+      let options = {includePrivateData: true};
+      TabState.copyFromCache(browser, tabState, options);
+
+      tabState.index += aDelta;
+      tabState.index = Math.max(1, Math.min(tabState.index, tabState.entries.length));
+      tabState.pinned = false;
+
+      // Restore the state into the new tab.
+      this.restoreTab(newTab, tabState, {
+        restoreImmediately: aRestoreImmediately
+      });
+    });
+
+    return newTab;
+  },
+
+  getClosedTabCount: function ssi_getClosedTabCount(aWindow) {
+    if ("__SSi" in aWindow) {
+      return this._windows[aWindow.__SSi]._closedTabs.length;
+    }
+
+    if (!DyingWindowCache.has(aWindow)) {
+      throw Components.Exception("Window is not tracked", Cr.NS_ERROR_INVALID_ARG);
+    }
+
+    return DyingWindowCache.get(aWindow)._closedTabs.length;
+  },
+
+  getClosedTabData: function ssi_getClosedTabData(aWindow, aAsString = true) {
+    if ("__SSi" in aWindow) {
+      return aAsString ?
+        JSON.stringify(this._windows[aWindow.__SSi]._closedTabs) :
+        Cu.cloneInto(this._windows[aWindow.__SSi]._closedTabs, {});
+    }
+
+    if (!DyingWindowCache.has(aWindow)) {
+      throw Components.Exception("Window is not tracked", Cr.NS_ERROR_INVALID_ARG);
+    }
+
+    let data = DyingWindowCache.get(aWindow);
+    return aAsString ? JSON.stringify(data._closedTabs) : Cu.cloneInto(data._closedTabs, {});
+  },
+
+  undoCloseTab: function ssi_undoCloseTab(aWindow, aIndex) {
+    if (!aWindow.__SSi) {
+      throw Components.Exception("Window is not tracked", Cr.NS_ERROR_INVALID_ARG);
+    }
+
+    var closedTabs = this._windows[aWindow.__SSi]._closedTabs;
+
+    // default to the most-recently closed tab
+    aIndex = aIndex || 0;
+    if (!(aIndex in closedTabs)) {
+      throw Components.Exception("Invalid index: not in the closed tabs", Cr.NS_ERROR_INVALID_ARG);
+    }
+
+    // fetch the data of closed tab, while removing it from the array
+    let {state, pos} = this.removeClosedTabData(closedTabs, aIndex);
+
+    // create a new tab
+    let tabbrowser = aWindow.gBrowser;
+    let tab = tabbrowser.selectedTab = tabbrowser.addTab(null, state);
+
+    // restore tab content
+    this.restoreTab(tab, state);
+
+    // restore the tab's position
+    tabbrowser.moveTabTo(tab, pos);
+
+    // focus the tab's content area (bug 342432)
+    tab.linkedBrowser.focus();
+
+    // Notify of changes to closed objects.
+    this._notifyOfClosedObjectsChange();
+
+    return tab;
+  },
+
+  forgetClosedTab: function ssi_forgetClosedTab(aWindow, aIndex) {
+    if (!aWindow.__SSi) {
+      throw Components.Exception("Window is not tracked", Cr.NS_ERROR_INVALID_ARG);
+    }
+
+    var closedTabs = this._windows[aWindow.__SSi]._closedTabs;
+
+    // default to the most-recently closed tab
+    aIndex = aIndex || 0;
+    if (!(aIndex in closedTabs)) {
+      throw Components.Exception("Invalid index: not in the closed tabs", Cr.NS_ERROR_INVALID_ARG);
+    }
+
+    // remove closed tab from the array
+    this.removeClosedTabData(closedTabs, aIndex);
+
+    // Notify of changes to closed objects.
+    this._notifyOfClosedObjectsChange();
+  },
+
+  getClosedWindowCount: function ssi_getClosedWindowCount() {
+    return this._closedWindows.length;
+  },
+
+  getClosedWindowData: function ssi_getClosedWindowData(aAsString = true) {
+    return aAsString ? JSON.stringify(this._closedWindows) : Cu.cloneInto(this._closedWindows, {});
+  },
+
+  undoCloseWindow: function ssi_undoCloseWindow(aIndex) {
+    if (!(aIndex in this._closedWindows)) {
+      throw Components.Exception("Invalid index: not in the closed windows", Cr.NS_ERROR_INVALID_ARG);
+    }
+
+    // reopen the window
+    let state = { windows: this._removeClosedWindow(aIndex) };
+    delete state.windows[0].closedAt; // Window is now open.
+
+    let window = this._openWindowWithState(state);
+    this.windowToFocus = window;
+
+    // Notify of changes to closed objects.
+    this._notifyOfClosedObjectsChange();
+
+    return window;
+  },
+
+  forgetClosedWindow: function ssi_forgetClosedWindow(aIndex) {
+    // default to the most-recently closed window
+    aIndex = aIndex || 0;
+    if (!(aIndex in this._closedWindows)) {
+      throw Components.Exception("Invalid index: not in the closed windows", Cr.NS_ERROR_INVALID_ARG);
+    }
+
+    // remove closed window from the array
+    let winData = this._closedWindows[aIndex];
+    this._removeClosedWindow(aIndex);
+    this._saveableClosedWindowData.delete(winData);
+
+    // Notify of changes to closed objects.
+    this._notifyOfClosedObjectsChange();
+  },
+
+  getWindowValue: function ssi_getWindowValue(aWindow, aKey) {
+    if ("__SSi" in aWindow) {
+      var data = this._windows[aWindow.__SSi].extData || {};
+      return data[aKey] || "";
+    }
+
+    if (DyingWindowCache.has(aWindow)) {
+      let data = DyingWindowCache.get(aWindow).extData || {};
+      return data[aKey] || "";
+    }
+
+    throw Components.Exception("Window is not tracked", Cr.NS_ERROR_INVALID_ARG);
+  },
+
+  setWindowValue: function ssi_setWindowValue(aWindow, aKey, aStringValue) {
+    if (typeof aStringValue != "string") {
+      throw new TypeError("setWindowValue only accepts string values");
+    }
+
+    if (!("__SSi" in aWindow)) {
+      throw Components.Exception("Window is not tracked", Cr.NS_ERROR_INVALID_ARG);
+    }
+    if (!this._windows[aWindow.__SSi].extData) {
+      this._windows[aWindow.__SSi].extData = {};
+    }
+    this._windows[aWindow.__SSi].extData[aKey] = aStringValue;
+    this.saveStateDelayed(aWindow);
+  },
+
+  deleteWindowValue: function ssi_deleteWindowValue(aWindow, aKey) {
+    if (aWindow.__SSi && this._windows[aWindow.__SSi].extData &&
+        this._windows[aWindow.__SSi].extData[aKey])
+      delete this._windows[aWindow.__SSi].extData[aKey];
+    this.saveStateDelayed(aWindow);
+  },
+
+  getTabValue: function ssi_getTabValue(aTab, aKey) {
+    return (aTab.__SS_extdata || {})[aKey] || "";
+  },
+
+  setTabValue: function ssi_setTabValue(aTab, aKey, aStringValue) {
+    if (typeof aStringValue != "string") {
+      throw new TypeError("setTabValue only accepts string values");
+    }
+
+    // If the tab hasn't been restored, then set the data there, otherwise we
+    // could lose newly added data.
+    if (!aTab.__SS_extdata) {
+      aTab.__SS_extdata = {};
+    }
+
+    aTab.__SS_extdata[aKey] = aStringValue;
+    this.saveStateDelayed(aTab.ownerGlobal);
+  },
+
+  deleteTabValue: function ssi_deleteTabValue(aTab, aKey) {
+    if (aTab.__SS_extdata && aKey in aTab.__SS_extdata) {
+      delete aTab.__SS_extdata[aKey];
+      this.saveStateDelayed(aTab.ownerGlobal);
+    }
+  },
+
+  getGlobalValue: function ssi_getGlobalValue(aKey) {
+    return this._globalState.get(aKey);
+  },
+
+  setGlobalValue: function ssi_setGlobalValue(aKey, aStringValue) {
+    if (typeof aStringValue != "string") {
+      throw new TypeError("setGlobalValue only accepts string values");
+    }
+
+    this._globalState.set(aKey, aStringValue);
+    this.saveStateDelayed();
+  },
+
+  deleteGlobalValue: function ssi_deleteGlobalValue(aKey) {
+    this._globalState.delete(aKey);
+    this.saveStateDelayed();
+  },
+
+  persistTabAttribute: function ssi_persistTabAttribute(aName) {
+    if (TabAttributes.persist(aName)) {
+      this.saveStateDelayed();
+    }
+  },
+
+
+  /**
+   * Undoes the closing of a tab or window which corresponds
+   * to the closedId passed in.
+   *
+   * @param aClosedId
+   *        The closedId of the tab or window
+   *
+   * @returns a tab or window object
+   */
+  undoCloseById(aClosedId) {
+    // Check for a window first.
+    for (let i = 0, l = this._closedWindows.length; i < l; i++) {
+      if (this._closedWindows[i].closedId == aClosedId) {
+        return this.undoCloseWindow(i);
+      }
+    }
+
+    // Check for a tab.
+    let windowsEnum = Services.wm.getEnumerator("navigator:browser");
+    while (windowsEnum.hasMoreElements()) {
+      let window = windowsEnum.getNext();
+      let windowState = this._windows[window.__SSi];
+      if (windowState) {
+        for (let j = 0, l = windowState._closedTabs.length; j < l; j++) {
+          if (windowState._closedTabs[j].closedId == aClosedId) {
+            return this.undoCloseTab(window, j);
+          }
+        }
+      }
+    }
+
+    // Neither a tab nor a window was found, return undefined and let the caller decide what to do about it.
+    return undefined;
+  },
+
+  /**
+   * Restores the session state stored in LastSession. This will attempt
+   * to merge data into the current session. If a window was opened at startup
+   * with pinned tab(s), then the remaining data from the previous session for
+   * that window will be opened into that window. Otherwise new windows will
+   * be opened.
+   */
+  restoreLastSession: function ssi_restoreLastSession() {
+    // Use the public getter since it also checks PB mode
+    if (!this.canRestoreLastSession) {
+      throw Components.Exception("Last session can not be restored");
+    }
+
+    Services.obs.notifyObservers(null, NOTIFY_INITIATING_MANUAL_RESTORE, "");
+
+    // First collect each window with its id...
+    let windows = {};
+    this._forEachBrowserWindow(function(aWindow) {
+      if (aWindow.__SS_lastSessionWindowID)
+        windows[aWindow.__SS_lastSessionWindowID] = aWindow;
+    });
+
+    let lastSessionState = LastSession.getState();
+
+    // This shouldn't ever be the case...
+    if (!lastSessionState.windows.length) {
+      throw Components.Exception("lastSessionState has no windows", Cr.NS_ERROR_UNEXPECTED);
+    }
+
+    // We're technically doing a restore, so set things up so we send the
+    // notification when we're done. We want to send "sessionstore-browser-state-restored".
+    this._restoreCount = lastSessionState.windows.length;
+    this._browserSetState = true;
+
+    // We want to re-use the last opened window instead of opening a new one in
+    // the case where it's "empty" and not associated with a window in the session.
+    // We will do more processing via _prepWindowToRestoreInto if we need to use
+    // the lastWindow.
+    let lastWindow = this._getMostRecentBrowserWindow();
+    let canUseLastWindow = lastWindow &&
+                           !lastWindow.__SS_lastSessionWindowID;
+
+    // global data must be restored before restoreWindow is called so that
+    // it happens before observers are notified
+    this._globalState.setFromState(lastSessionState);
+
+    // Restore into windows or open new ones as needed.
+    for (let i = 0; i < lastSessionState.windows.length; i++) {
+      let winState = lastSessionState.windows[i];
+      let lastSessionWindowID = winState.__lastSessionWindowID;
+      // delete lastSessionWindowID so we don't add that to the window again
+      delete winState.__lastSessionWindowID;
+
+      // See if we can use an open window. First try one that is associated with
+      // the state we're trying to restore and then fallback to the last selected
+      // window.
+      let windowToUse = windows[lastSessionWindowID];
+      if (!windowToUse && canUseLastWindow) {
+        windowToUse = lastWindow;
+        canUseLastWindow = false;
+      }
+
+      let [canUseWindow, canOverwriteTabs] = this._prepWindowToRestoreInto(windowToUse);
+
+      // If there's a window already open that we can restore into, use that
+      if (canUseWindow) {
+        // Since we're not overwriting existing tabs, we want to merge _closedTabs,
+        // putting existing ones first. Then make sure we're respecting the max pref.
+        if (winState._closedTabs && winState._closedTabs.length) {
+          let curWinState = this._windows[windowToUse.__SSi];
+          curWinState._closedTabs = curWinState._closedTabs.concat(winState._closedTabs);
+          curWinState._closedTabs.splice(this._max_tabs_undo, curWinState._closedTabs.length);
+        }
+
+        // Restore into that window - pretend it's a followup since we'll already
+        // have a focused window.
+        //XXXzpao This is going to merge extData together (taking what was in
+        //        winState over what is in the window already.
+        let options = {overwriteTabs: canOverwriteTabs, isFollowUp: true};
+        this.restoreWindow(windowToUse, winState, options);
+      }
+      else {
+        this._openWindowWithState({ windows: [winState] });
+      }
+    }
+
+    // Merge closed windows from this session with ones from last session
+    if (lastSessionState._closedWindows) {
+      this._closedWindows = this._closedWindows.concat(lastSessionState._closedWindows);
+      this._capClosedWindows();
+      this._closedObjectsChanged = true;
+    }
+
+    // Scratchpad
+    if (lastSessionState.scratchpads) {
+      ScratchpadManager.restoreSession(lastSessionState.scratchpads);
+    }
+
+    // The Browser Console
+    if (lastSessionState.browserConsole) {
+      HUDService.restoreBrowserConsoleSession();
+    }
+
+    // Set data that persists between sessions
+    this._recentCrashes = lastSessionState.session &&
+                          lastSessionState.session.recentCrashes || 0;
+
+    // Update the session start time using the restored session state.
+    this._updateSessionStartTime(lastSessionState);
+
+    LastSession.clear();
+
+    // Notify of changes to closed objects.
+    this._notifyOfClosedObjectsChange();
+  },
+
+  /**
+   * Revive a crashed tab and restore its state from before it crashed.
+   *
+   * @param aTab
+   *        A <xul:tab> linked to a crashed browser. This is a no-op if the
+   *        browser hasn't actually crashed, or is not associated with a tab.
+   *        This function will also throw if the browser happens to be remote.
+   */
+  reviveCrashedTab(aTab) {
+    if (!aTab) {
+      throw new Error("SessionStore.reviveCrashedTab expected a tab, but got null.");
+    }
+
+    let browser = aTab.linkedBrowser;
+    if (!this._crashedBrowsers.has(browser.permanentKey)) {
+      return;
+    }
+
+    // Sanity check - the browser to be revived should not be remote
+    // at this point.
+    if (browser.isRemoteBrowser) {
+      throw new Error("SessionStore.reviveCrashedTab: " +
+                      "Somehow a crashed browser is still remote.")
+    }
+
+    // We put the browser at about:blank in case the user is
+    // restoring tabs on demand. This way, the user won't see
+    // a flash of the about:tabcrashed page after selecting
+    // the revived tab.
+    aTab.removeAttribute("crashed");
+    browser.loadURI("about:blank", null, null);
+
+    let data = TabState.collect(aTab);
+    this.restoreTab(aTab, data, {
+      forceOnDemand: true,
+    });
+  },
+
+  /**
+   * Revive all crashed tabs and reset the crashed tabs count to 0.
+   */
+  reviveAllCrashedTabs() {
+    let windowsEnum = Services.wm.getEnumerator("navigator:browser");
+    while (windowsEnum.hasMoreElements()) {
+      let window = windowsEnum.getNext();
+      for (let tab of window.gBrowser.tabs) {
+        this.reviveCrashedTab(tab);
+      }
+    }
+  },
+
+  /**
+   * Navigate the given |tab| by first collecting its current state and then
+   * either changing only the index of the currently shown history entry,
+   * or restoring the exact same state again and passing the new URL to load
+   * in |loadArguments|. Use this method to seamlessly switch between pages
+   * loaded in the parent and pages loaded in the child process.
+   *
+   * This method might be called multiple times before it has finished
+   * flushing the browser tab. If that occurs, the loadArguments from
+   * the most recent call to navigateAndRestore will be used once the
+   * flush has finished.
+   */
+  navigateAndRestore(tab, loadArguments, historyIndex) {
+    let window = tab.ownerGlobal;
+    NS_ASSERT(window.__SSi, "tab's window must be tracked");
+    let browser = tab.linkedBrowser;
+
+    // Were we already waiting for a flush from a previous call to
+    // navigateAndRestore on this tab?
+    let alreadyRestoring =
+      this._remotenessChangingBrowsers.has(browser.permanentKey);
+
+    // Stash the most recent loadArguments in this WeakMap so that
+    // we know to use it when the TabStateFlusher.flush resolves.
+    this._remotenessChangingBrowsers.set(browser.permanentKey, loadArguments);
+
+    if (alreadyRestoring) {
+      // This tab was already being restored to run in the
+      // correct process. We're done here.
+      return;
+    }
+
+    // Set tab title to "Connecting..." and start the throbber to pretend we're
+    // doing something while actually waiting for data from the frame script.
+    window.gBrowser.setTabTitleLoading(tab);
+    tab.setAttribute("busy", "true");
+
+    // Flush to get the latest tab state.
+    TabStateFlusher.flush(browser).then(() => {
+      // loadArguments might have been overwritten by multiple calls
+      // to navigateAndRestore while we waited for the tab to flush,
+      // so we use the most recently stored one.
+      let recentLoadArguments =
+        this._remotenessChangingBrowsers.get(browser.permanentKey);
+      this._remotenessChangingBrowsers.delete(browser.permanentKey);
+
+      // The tab might have been closed/gone in the meantime.
+      if (tab.closing || !tab.linkedBrowser) {
+        return;
+      }
+
+      let window = tab.ownerGlobal;
+
+      // The tab or its window might be gone.
+      if (!window || !window.__SSi || window.closed) {
+        return;
+      }
+
+      let tabState = TabState.clone(tab);
+      let options = {
+        restoreImmediately: true,
+        // We want to make sure that this information is passed to restoreTab
+        // whether or not a historyIndex is passed in. Thus, we extract it from
+        // the loadArguments.
+        reloadInFreshProcess: !!recentLoadArguments.reloadInFreshProcess,
+      };
+
+      if (historyIndex >= 0) {
+        tabState.index = historyIndex + 1;
+        tabState.index = Math.max(1, Math.min(tabState.index, tabState.entries.length));
+      } else {
+        options.loadArguments = recentLoadArguments;
+      }
+
+      // Need to reset restoring tabs.
+      if (tab.linkedBrowser.__SS_restoreState) {
+        this._resetLocalTabRestoringState(tab);
+      }
+
+      // Restore the state into the tab.
+      this.restoreTab(tab, tabState, options);
+    });
+
+    tab.linkedBrowser.__SS_restoreState = TAB_STATE_WILL_RESTORE;
+
+    // Notify of changes to closed objects.
+    this._notifyOfClosedObjectsChange();
+  },
+
+  /**
+   * Retrieves the latest session history information for a tab. The cached data
+   * is returned immediately, but a callback may be provided that supplies
+   * up-to-date data when or if it is available. The callback is passed a single
+   * argument with data in the same format as the return value.
+   *
+   * @param tab tab to retrieve the session history for
+   * @param updatedCallback function to call with updated data as the single argument
+   * @returns a object containing 'index' specifying the current index, and an
+   * array 'entries' containing an object for each history item.
+   */
+  getSessionHistory(tab, updatedCallback) {
+    if (updatedCallback) {
+      TabStateFlusher.flush(tab.linkedBrowser).then(() => {
+        let sessionHistory = this.getSessionHistory(tab);
+        if (sessionHistory) {
+          updatedCallback(sessionHistory);
+        }
+      });
+    }
+
+    // Don't continue if the tab was closed before TabStateFlusher.flush resolves.
+    if (tab.linkedBrowser) {
+      let tabState = TabState.collect(tab);
+      return { index: tabState.index - 1, entries: tabState.entries }
+    }
+  },
+
+  /**
+   * See if aWindow is usable for use when restoring a previous session via
+   * restoreLastSession. If usable, prepare it for use.
+   *
+   * @param aWindow
+   *        the window to inspect & prepare
+   * @returns [canUseWindow, canOverwriteTabs]
+   *          canUseWindow: can the window be used to restore into
+   *          canOverwriteTabs: all of the current tabs are home pages and we
+   *                            can overwrite them
+   */
+  _prepWindowToRestoreInto: function ssi_prepWindowToRestoreInto(aWindow) {
+    if (!aWindow)
+      return [false, false];
+
+    // We might be able to overwrite the existing tabs instead of just adding
+    // the previous session's tabs to the end. This will be set if possible.
+    let canOverwriteTabs = false;
+
+    // Look at the open tabs in comparison to home pages. If all the tabs are
+    // home pages then we'll end up overwriting all of them. Otherwise we'll
+    // just close the tabs that match home pages. Tabs with the about:blank
+    // URI will always be overwritten.
+    let homePages = ["about:blank"];
+    let removableTabs = [];
+    let tabbrowser = aWindow.gBrowser;
+    let normalTabsLen = tabbrowser.tabs.length - tabbrowser._numPinnedTabs;
+    let startupPref = this._prefBranch.getIntPref("startup.page");
+    if (startupPref == 1)
+      homePages = homePages.concat(aWindow.gHomeButton.getHomePage().split("|"));
+
+    for (let i = tabbrowser._numPinnedTabs; i < tabbrowser.tabs.length; i++) {
+      let tab = tabbrowser.tabs[i];
+      if (homePages.indexOf(tab.linkedBrowser.currentURI.spec) != -1) {
+        removableTabs.push(tab);
+      }
+    }
+
+    if (tabbrowser.tabs.length == removableTabs.length) {
+      canOverwriteTabs = true;
+    }
+    else {
+      // If we're not overwriting all of the tabs, then close the home tabs.
+      for (let i = removableTabs.length - 1; i >= 0; i--) {
+        tabbrowser.removeTab(removableTabs.pop(), { animate: false });
+      }
+    }
+
+    return [true, canOverwriteTabs];
+  },
+
+  /* ........ Saving Functionality .............. */
+
+  /**
+   * Store window dimensions, visibility, sidebar
+   * @param aWindow
+   *        Window reference
+   */
+  _updateWindowFeatures: function ssi_updateWindowFeatures(aWindow) {
+    var winData = this._windows[aWindow.__SSi];
+
+    WINDOW_ATTRIBUTES.forEach(function(aAttr) {
+      winData[aAttr] = this._getWindowDimension(aWindow, aAttr);
+    }, this);
+
+    var hidden = WINDOW_HIDEABLE_FEATURES.filter(function(aItem) {
+      return aWindow[aItem] && !aWindow[aItem].visible;
+    });
+    if (hidden.length != 0)
+      winData.hidden = hidden.join(",");
+    else if (winData.hidden)
+      delete winData.hidden;
+
+    var sidebar = aWindow.document.getElementById("sidebar-box").getAttribute("sidebarcommand");
+    if (sidebar)
+      winData.sidebar = sidebar;
+    else if (winData.sidebar)
+      delete winData.sidebar;
+  },
+
+  /**
+   * gather session data as object
+   * @param aUpdateAll
+   *        Bool update all windows
+   * @returns object
+   */
+  getCurrentState: function (aUpdateAll) {
+    this._handleClosedWindows().then(() => {
+      this._notifyOfClosedObjectsChange();
+    });
+
+    var activeWindow = this._getMostRecentBrowserWindow();
+
+    TelemetryStopwatch.start("FX_SESSION_RESTORE_COLLECT_ALL_WINDOWS_DATA_MS");
+    if (RunState.isRunning) {
+      // update the data for all windows with activities since the last save operation
+      this._forEachBrowserWindow(function(aWindow) {
+        if (!this._isWindowLoaded(aWindow)) // window data is still in _statesToRestore
+          return;
+        if (aUpdateAll || DirtyWindows.has(aWindow) || aWindow == activeWindow) {
+          this._collectWindowData(aWindow);
+        }
+        else { // always update the window features (whose change alone never triggers a save operation)
+          this._updateWindowFeatures(aWindow);
+        }
+      });
+      DirtyWindows.clear();
+    }
+    TelemetryStopwatch.finish("FX_SESSION_RESTORE_COLLECT_ALL_WINDOWS_DATA_MS");
+
+    // An array that at the end will hold all current window data.
+    var total = [];
+    // The ids of all windows contained in 'total' in the same order.
+    var ids = [];
+    // The number of window that are _not_ popups.
+    var nonPopupCount = 0;
+    var ix;
+
+    // collect the data for all windows
+    for (ix in this._windows) {
+      if (this._windows[ix]._restoring) // window data is still in _statesToRestore
+        continue;
+      total.push(this._windows[ix]);
+      ids.push(ix);
+      if (!this._windows[ix].isPopup)
+        nonPopupCount++;
+    }
+
+    TelemetryStopwatch.start("FX_SESSION_RESTORE_COLLECT_COOKIES_MS");
+    SessionCookies.update(total);
+    TelemetryStopwatch.finish("FX_SESSION_RESTORE_COLLECT_COOKIES_MS");
+
+    // collect the data for all windows yet to be restored
+    for (ix in this._statesToRestore) {
+      for (let winData of this._statesToRestore[ix].windows) {
+        total.push(winData);
+        if (!winData.isPopup)
+          nonPopupCount++;
+      }
+    }
+
+    // shallow copy this._closedWindows to preserve current state
+    let lastClosedWindowsCopy = this._closedWindows.slice();
+
+#ifndef XP_MACOSX
+    // If no non-popup browser window remains open, return the state of the last
+    // closed window(s). We only want to do this when we're actually "ending"
+    // the session.
+    //XXXzpao We should do this for _restoreLastWindow == true, but that has
+    //        its own check for popups. c.f. bug 597619
+    if (nonPopupCount == 0 && lastClosedWindowsCopy.length > 0 &&
+        RunState.isQuitting) {
+      // prepend the last non-popup browser window, so that if the user loads more tabs
+      // at startup we don't accidentally add them to a popup window
+      do {
+        total.unshift(lastClosedWindowsCopy.shift())
+      } while (total[0].isPopup && lastClosedWindowsCopy.length > 0)
+    }
+#endif
+
+    if (activeWindow) {
+      this.activeWindowSSiCache = activeWindow.__SSi || "";
+    }
+    ix = ids.indexOf(this.activeWindowSSiCache);
+    // We don't want to restore focus to a minimized window or a window which had all its
+    // tabs stripped out (doesn't exist).
+    if (ix != -1 && total[ix] && total[ix].sizemode == "minimized")
+      ix = -1;
+
+    let session = {
+      lastUpdate: Date.now(),
+      startTime: this._sessionStartTime,
+      recentCrashes: this._recentCrashes
+    };
+
+    let state = {
+      version: ["sessionrestore", FORMAT_VERSION],
+      windows: total,
+      selectedWindow: ix + 1,
+      _closedWindows: lastClosedWindowsCopy,
+      session: session,
+      global: this._globalState.getState()
+    };
+
+    // Scratchpad
+    if (Cu.isModuleLoaded("resource://devtools/client/scratchpad/scratchpad-manager.jsm")) {
+      // get open Scratchpad window states too
+      let scratchpads = ScratchpadManager.getSessionState();
+      if (scratchpads && scratchpads.length) {
+        state.scratchpads = scratchpads;
+      }
+    }
+
+    // The Browser Console
+    state.browserConsole = HUDService.getBrowserConsoleSessionState();
+
+    // Persist the last session if we deferred restoring it
+    if (LastSession.canRestore) {
+      state.lastSessionState = LastSession.getState();
+    }
+
+    // If we were called by the SessionSaver and started with only a private
+    // window we want to pass the deferred initial state to not lose the
+    // previous session.
+    if (this._deferredInitialState) {
+      state.deferredInitialState = this._deferredInitialState;
+    }
+
+    return state;
+  },
+
+  /**
+   * serialize session data for a window
+   * @param aWindow
+   *        Window reference
+   * @returns string
+   */
+  _getWindowState: function ssi_getWindowState(aWindow) {
+    if (!this._isWindowLoaded(aWindow))
+      return this._statesToRestore[aWindow.__SS_restoreID];
+
+    if (RunState.isRunning) {
+      this._collectWindowData(aWindow);
+    }
+
+    let windows = [this._windows[aWindow.__SSi]];
+    SessionCookies.update(windows);
+
+    return { windows: windows };
+  },
+
+  /**
+   * Gathers data about a window and its tabs, and updates its
+   * entry in this._windows.
+   *
+   * @param aWindow
+   *        Window references.
+   * @returns a Map mapping the browser tabs from aWindow to the tab
+   *          entry that was put into the window data in this._windows.
+   */
+  _collectWindowData: function ssi_collectWindowData(aWindow) {
+    let tabMap = new Map();
+
+    if (!this._isWindowLoaded(aWindow))
+      return tabMap;
+
+    let tabbrowser = aWindow.gBrowser;
+    let tabs = tabbrowser.tabs;
+    let winData = this._windows[aWindow.__SSi];
+    let tabsData = winData.tabs = [];
+
+    // update the internal state data for this window
+    for (let tab of tabs) {
+      let tabData = TabState.collect(tab);
+      tabMap.set(tab, tabData);
+      tabsData.push(tabData);
+    }
+    winData.selected = tabbrowser.mTabBox.selectedIndex + 1;
+
+    this._updateWindowFeatures(aWindow);
+
+    // Make sure we keep __SS_lastSessionWindowID around for cases like entering
+    // or leaving PB mode.
+    if (aWindow.__SS_lastSessionWindowID)
+      this._windows[aWindow.__SSi].__lastSessionWindowID =
+        aWindow.__SS_lastSessionWindowID;
+
+    DirtyWindows.remove(aWindow);
+    return tabMap;
+  },
+
+  /* ........ Restoring Functionality .............. */
+
+  /**
+   * restore features to a single window
+   * @param aWindow
+   *        Window reference to the window to use for restoration
+   * @param winData
+   *        JS object
+   * @param aOptions
+   *        {overwriteTabs: true} to overwrite existing tabs w/ new ones
+   *        {isFollowUp: true} if this is not the restoration of the 1st window
+   *        {firstWindow: true} if this is the first non-private window we're
+   *                            restoring in this session, that might open an
+   *                            external link as well
+   */
+  restoreWindow: function ssi_restoreWindow(aWindow, winData, aOptions = {}) {
+    let overwriteTabs = aOptions && aOptions.overwriteTabs;
+    let isFollowUp = aOptions && aOptions.isFollowUp;
+    let firstWindow = aOptions && aOptions.firstWindow;
+
+    if (isFollowUp) {
+      this.windowToFocus = aWindow;
+    }
+
+    // initialize window if necessary
+    if (aWindow && (!aWindow.__SSi || !this._windows[aWindow.__SSi]))
+      this.onLoad(aWindow);
+
+    TelemetryStopwatch.start("FX_SESSION_RESTORE_RESTORE_WINDOW_MS");
+
+    // We're not returning from this before we end up calling restoreTabs
+    // for this window, so make sure we send the SSWindowStateBusy event.
+    this._setWindowStateBusy(aWindow);
+
+    if (!winData.tabs) {
+      winData.tabs = [];
+    }
+
+    // don't restore a single blank tab when we've had an external
+    // URL passed in for loading at startup (cf. bug 357419)
+    else if (firstWindow && !overwriteTabs && winData.tabs.length == 1 &&
+             (!winData.tabs[0].entries || winData.tabs[0].entries.length == 0)) {
+      winData.tabs = [];
+    }
+
+    var tabbrowser = aWindow.gBrowser;
+    var openTabCount = overwriteTabs ? tabbrowser.browsers.length : -1;
+    var newTabCount = winData.tabs.length;
+    var tabs = [];
+
+    // disable smooth scrolling while adding, moving, removing and selecting tabs
+    var tabstrip = tabbrowser.tabContainer.mTabstrip;
+    var smoothScroll = tabstrip.smoothScroll;
+    tabstrip.smoothScroll = false;
+
+    // unpin all tabs to ensure they are not reordered in the next loop
+    if (overwriteTabs) {
+      for (let t = tabbrowser._numPinnedTabs - 1; t > -1; t--)
+        tabbrowser.unpinTab(tabbrowser.tabs[t]);
+    }
+
+    // We need to keep track of the initially open tabs so that they
+    // can be moved to the end of the restored tabs.
+    let initialTabs = [];
+    if (!overwriteTabs && firstWindow) {
+      initialTabs = Array.slice(tabbrowser.tabs);
+    }
+
+    // make sure that the selected tab won't be closed in order to
+    // prevent unnecessary flickering
+    if (overwriteTabs && tabbrowser.selectedTab._tPos >= newTabCount)
+      tabbrowser.moveTabTo(tabbrowser.selectedTab, newTabCount - 1);
+
+    let numVisibleTabs = 0;
+
+    for (var t = 0; t < newTabCount; t++) {
+      // When trying to restore into existing tab, we also take the userContextId
+      // into account if present.
+      let userContextId = winData.tabs[t].userContextId;
+      let reuseExisting = t < openTabCount &&
+                          (tabbrowser.tabs[t].getAttribute("usercontextid") == (userContextId || ""));
+      // If the tab is pinned, then we'll be loading it right away, and
+      // there's no need to cause a remoteness flip by loading it initially
+      // non-remote.
+      let forceNotRemote = !winData.tabs[t].pinned;
+      let tab = reuseExisting ? tabbrowser.tabs[t] :
+                                tabbrowser.addTab("about:blank",
+                                                  {skipAnimation: true,
+                                                   forceNotRemote,
+                                                   userContextId});
+
+      // If we inserted a new tab because the userContextId didn't match with the
+      // open tab, even though `t < openTabCount`, we need to remove that open tab
+      // and put the newly added tab in its place.
+      if (!reuseExisting && t < openTabCount) {
+        tabbrowser.removeTab(tabbrowser.tabs[t]);
+        tabbrowser.moveTabTo(tab, t);
+      }
+
+      tabs.push(tab);
+
+      if (winData.tabs[t].pinned)
+        tabbrowser.pinTab(tabs[t]);
+
+      if (winData.tabs[t].hidden) {
+        tabbrowser.hideTab(tabs[t]);
+      }
+      else {
+        tabbrowser.showTab(tabs[t]);
+        numVisibleTabs++;
+      }
+
+      if (!!winData.tabs[t].muted != tabs[t].linkedBrowser.audioMuted) {
+        tabs[t].toggleMuteAudio(winData.tabs[t].muteReason);
+      }
+    }
+
+    if (!overwriteTabs && firstWindow) {
+      // Move the originally open tabs to the end
+      let endPosition = tabbrowser.tabs.length - 1;
+      for (let i = 0; i < initialTabs.length; i++) {
+        tabbrowser.moveTabTo(initialTabs[i], endPosition);
+      }
+    }
+
+    // if all tabs to be restored are hidden, make the first one visible
+    if (!numVisibleTabs && winData.tabs.length) {
+      winData.tabs[0].hidden = false;
+      tabbrowser.showTab(tabs[0]);
+    }
+
+    // If overwriting tabs, we want to reset each tab's "restoring" state. Since
+    // we're overwriting those tabs, they should no longer be restoring. The
+    // tabs will be rebuilt and marked if they need to be restored after loading
+    // state (in restoreTabs).
+    if (overwriteTabs) {
+      for (let i = 0; i < tabbrowser.tabs.length; i++) {
+        let tab = tabbrowser.tabs[i];
+        if (tabbrowser.browsers[i].__SS_restoreState)
+          this._resetTabRestoringState(tab);
+      }
+    }
+
+    // We want to correlate the window with data from the last session, so
+    // assign another id if we have one. Otherwise clear so we don't do
+    // anything with it.
+    delete aWindow.__SS_lastSessionWindowID;
+    if (winData.__lastSessionWindowID)
+      aWindow.__SS_lastSessionWindowID = winData.__lastSessionWindowID;
+
+    // when overwriting tabs, remove all superflous ones
+    if (overwriteTabs && newTabCount < openTabCount) {
+      Array.slice(tabbrowser.tabs, newTabCount, openTabCount)
+           .forEach(tabbrowser.removeTab, tabbrowser);
+    }
+
+    if (overwriteTabs) {
+      this.restoreWindowFeatures(aWindow, winData);
+      delete this._windows[aWindow.__SSi].extData;
+    }
+    if (winData.cookies) {
+      SessionCookies.restore(winData.cookies);
+    }
+    if (winData.extData) {
+      if (!this._windows[aWindow.__SSi].extData) {
+        this._windows[aWindow.__SSi].extData = {};
+      }
+      for (var key in winData.extData) {
+        this._windows[aWindow.__SSi].extData[key] = winData.extData[key];
+      }
+    }
+
+    let newClosedTabsData = winData._closedTabs || [];
+
+    if (overwriteTabs || firstWindow) {
+      // Overwrite existing closed tabs data when overwriteTabs=true
+      // or we're the first window to be restored.
+      this._windows[aWindow.__SSi]._closedTabs = newClosedTabsData;
+    } else if (this._max_tabs_undo > 0) {
+      // If we merge tabs, we also want to merge closed tabs data. We'll assume
+      // the restored tabs were closed more recently and append the current list
+      // of closed tabs to the new one...
+      newClosedTabsData =
+        newClosedTabsData.concat(this._windows[aWindow.__SSi]._closedTabs);
+
+      // ... and make sure that we don't exceed the max number of closed tabs
+      // we can restore.
+      this._windows[aWindow.__SSi]._closedTabs =
+        newClosedTabsData.slice(0, this._max_tabs_undo);
+    }
+
+    // Restore tabs, if any.
+    if (winData.tabs.length) {
+      this.restoreTabs(aWindow, tabs, winData.tabs,
+        (overwriteTabs ? (parseInt(winData.selected || "1")) : 0));
+    }
+
+    // set smoothScroll back to the original value
+    tabstrip.smoothScroll = smoothScroll;
+
+    TelemetryStopwatch.finish("FX_SESSION_RESTORE_RESTORE_WINDOW_MS");
+
+    this._setWindowStateReady(aWindow);
+
+    this._sendWindowRestoredNotification(aWindow);
+
+    Services.obs.notifyObservers(aWindow, NOTIFY_SINGLE_WINDOW_RESTORED, "");
+
+    this._sendRestoreCompletedNotifications();
+  },
+
+  /**
+   * Restore multiple windows using the provided state.
+   * @param aWindow
+   *        Window reference to the first window to use for restoration.
+   *        Additionally required windows will be opened.
+   * @param aState
+   *        JS object or JSON string
+   * @param aOptions
+   *        {overwriteTabs: true} to overwrite existing tabs w/ new ones
+   *        {isFollowUp: true} if this is not the restoration of the 1st window
+   *        {firstWindow: true} if this is the first non-private window we're
+   *                            restoring in this session, that might open an
+   *                            external link as well
+   */
+  restoreWindows: function ssi_restoreWindows(aWindow, aState, aOptions = {}) {
+    let isFollowUp = aOptions && aOptions.isFollowUp;
+
+    if (isFollowUp) {
+      this.windowToFocus = aWindow;
+    }
+
+    // initialize window if necessary
+    if (aWindow && (!aWindow.__SSi || !this._windows[aWindow.__SSi]))
+      this.onLoad(aWindow);
+
+    let root;
+    try {
+      root = (typeof aState == "string") ? JSON.parse(aState) : aState;
+    }
+    catch (ex) { // invalid state object - don't restore anything
+      debug(ex);
+      this._sendRestoreCompletedNotifications();
+      return;
+    }
+
+    // Restore closed windows if any.
+    if (root._closedWindows) {
+      this._closedWindows = root._closedWindows;
+      this._closedObjectsChanged = true;
+    }
+
+    // We're done here if there are no windows.
+    if (!root.windows || !root.windows.length) {
+      this._sendRestoreCompletedNotifications();
+      return;
+    }
+
+    if (!root.selectedWindow || root.selectedWindow > root.windows.length) {
+      root.selectedWindow = 0;
+    }
+
+    // open new windows for all further window entries of a multi-window session
+    // (unless they don't contain any tab data)
+    let winData;
+    for (var w = 1; w < root.windows.length; w++) {
+      winData = root.windows[w];
+      if (winData && winData.tabs && winData.tabs[0]) {
+        var window = this._openWindowWithState({ windows: [winData] });
+        if (w == root.selectedWindow - 1) {
+          this.windowToFocus = window;
+        }
+      }
+    }
+
+    this.restoreWindow(aWindow, root.windows[0], aOptions);
+
+    // Scratchpad
+    if (aState.scratchpads) {
+      ScratchpadManager.restoreSession(aState.scratchpads);
+    }
+
+    // The Browser Console
+    if (aState.browserConsole) {
+      HUDService.restoreBrowserConsoleSession();
+    }
+  },
+
+  /**
+   * Manage history restoration for a window
+   * @param aWindow
+   *        Window to restore the tabs into
+   * @param aTabs
+   *        Array of tab references
+   * @param aTabData
+   *        Array of tab data
+   * @param aSelectTab
+   *        Index of the tab to select. This is a 1-based index where "1"
+   *        indicates the first tab should be selected, and "0" indicates that
+   *        the currently selected tab will not be changed.
+   */
+  restoreTabs(aWindow, aTabs, aTabData, aSelectTab) {
+    var tabbrowser = aWindow.gBrowser;
+
+    if (!this._isWindowLoaded(aWindow)) {
+      // from now on, the data will come from the actual window
+      delete this._statesToRestore[aWindow.__SS_restoreID];
+      delete aWindow.__SS_restoreID;
+      delete this._windows[aWindow.__SSi]._restoring;
+    }
+
+    let numTabsToRestore = aTabs.length;
+    let numTabsInWindow = tabbrowser.tabs.length;
+    let tabsDataArray = this._windows[aWindow.__SSi].tabs;
+
+    // Update the window state in case we shut down without being notified.
+    // Individual tab states will be taken care of by restoreTab() below.
+    if (numTabsInWindow == numTabsToRestore) {
+      // Remove all previous tab data.
+      tabsDataArray.length = 0;
+    } else {
+      // Remove all previous tab data except tabs that should not be overriden.
+      tabsDataArray.splice(numTabsInWindow - numTabsToRestore);
+    }
+
+    // Let the tab data array have the right number of slots.
+    tabsDataArray.length = numTabsInWindow;
+
+    // If provided, set the selected tab.
+    if (aSelectTab > 0 && aSelectTab <= aTabs.length) {
+      tabbrowser.selectedTab = aTabs[aSelectTab - 1];
+
+      // Update the window state in case we shut down without being notified.
+      this._windows[aWindow.__SSi].selected = aSelectTab;
+    }
+
+    // Restore all tabs.
+    for (let t = 0; t < aTabs.length; t++) {
+      this.restoreTab(aTabs[t], aTabData[t]);
+    }
+  },
+
+  // Restores the given tab state for a given tab.
+  restoreTab(tab, tabData, options = {}) {
+    NS_ASSERT(!tab.linkedBrowser.__SS_restoreState,
+              "must reset tab before calling restoreTab()");
+
+    let restoreImmediately = options.restoreImmediately;
+    let loadArguments = options.loadArguments;
+    let browser = tab.linkedBrowser;
+    let window = tab.ownerGlobal;
+    let tabbrowser = window.gBrowser;
+    let forceOnDemand = options.forceOnDemand;
+    let reloadInFreshProcess = options.reloadInFreshProcess;
+
+    let willRestoreImmediately = restoreImmediately ||
+                                 tabbrowser.selectedBrowser == browser ||
+                                 loadArguments;
+
+    if (!willRestoreImmediately && !forceOnDemand) {
+      TabRestoreQueue.add(tab);
+    }
+
+    this._maybeUpdateBrowserRemoteness({ tabbrowser, tab,
+                                         willRestoreImmediately });
+
+    // Increase the busy state counter before modifying the tab.
+    this._setWindowStateBusy(window);
+
+    // It's important to set the window state to dirty so that
+    // we collect their data for the first time when saving state.
+    DirtyWindows.add(window);
+
+    // In case we didn't collect/receive data for any tabs yet we'll have to
+    // fill the array with at least empty tabData objects until |_tPos| or
+    // we'll end up with |null| entries.
+    for (let otherTab of Array.slice(tabbrowser.tabs, 0, tab._tPos)) {
+      let emptyState = {entries: [], lastAccessed: otherTab.lastAccessed};
+      this._windows[window.__SSi].tabs.push(emptyState);
+    }
+
+    // Update the tab state in case we shut down without being notified.
+    this._windows[window.__SSi].tabs[tab._tPos] = tabData;
+
+    // Prepare the tab so that it can be properly restored. We'll pin/unpin
+    // and show/hide tabs as necessary. We'll also attach a copy of the tab's
+    // data in case we close it before it's been restored.
+    if (tabData.pinned) {
+      tabbrowser.pinTab(tab);
+    } else {
+      tabbrowser.unpinTab(tab);
+    }
+
+    if (tabData.hidden) {
+      tabbrowser.hideTab(tab);
+    } else {
+      tabbrowser.showTab(tab);
+    }
+
+    if (!!tabData.muted != browser.audioMuted) {
+      tab.toggleMuteAudio(tabData.muteReason);
+    }
+
+    if (tabData.lastAccessed) {
+      tab.updateLastAccessed(tabData.lastAccessed);
+    }
+
+    if ("attributes" in tabData) {
+      // Ensure that we persist tab attributes restored from previous sessions.
+      Object.keys(tabData.attributes).forEach(a => TabAttributes.persist(a));
+    }
+
+    if (!tabData.entries) {
+      tabData.entries = [];
+    }
+    if (tabData.extData) {
+      tab.__SS_extdata = Cu.cloneInto(tabData.extData, {});
+    } else {
+      delete tab.__SS_extdata;
+    }
+
+    // Tab is now open.
+    delete tabData.closedAt;
+
+    // Ensure the index is in bounds.
+    let activeIndex = (tabData.index || tabData.entries.length) - 1;
+    activeIndex = Math.min(activeIndex, tabData.entries.length - 1);
+    activeIndex = Math.max(activeIndex, 0);
+
+    // Save the index in case we updated it above.
+    tabData.index = activeIndex + 1;
+
+    // Start a new epoch to discard all frame script messages relating to a
+    // previous epoch. All async messages that are still on their way to chrome
+    // will be ignored and don't override any tab data set when restoring.
+    let epoch = this.startNextEpoch(browser);
+
+    // keep the data around to prevent dataloss in case
+    // a tab gets closed before it's been properly restored
+    browser.__SS_restoreState = TAB_STATE_NEEDS_RESTORE;
+    browser.setAttribute("pending", "true");
+    tab.setAttribute("pending", "true");
+
+    // If we're restoring this tab, it certainly shouldn't be in
+    // the ignored set anymore.
+    this._crashedBrowsers.delete(browser.permanentKey);
+
+    // Update the persistent tab state cache with |tabData| information.
+    TabStateCache.update(browser, {
+      // NOTE: Copy the entries array shallowly, so as to not screw with the
+      // original tabData's history when getting history updates.
+      history: {entries: [...tabData.entries], index: tabData.index},
+      scroll: tabData.scroll || null,
+      storage: tabData.storage || null,
+      formdata: tabData.formdata || null,
+      disallow: tabData.disallow || null,
+      pageStyle: tabData.pageStyle || null,
+
+      // This information is only needed until the tab has finished restoring.
+      // When that's done it will be removed from the cache and we always
+      // collect it in TabState._collectBaseTabData().
+      image: tabData.image || "",
+      iconLoadingPrincipal: tabData.iconLoadingPrincipal || null,
+      userTypedValue: tabData.userTypedValue || "",
+      userTypedClear: tabData.userTypedClear || 0
+    });
+
+    browser.messageManager.sendAsyncMessage("SessionStore:restoreHistory",
+                                            {tabData: tabData, epoch: epoch, loadArguments});
+
+    // Restore tab attributes.
+    if ("attributes" in tabData) {
+      TabAttributes.set(tab, tabData.attributes);
+    }
+
+    // This could cause us to ignore MAX_CONCURRENT_TAB_RESTORES a bit, but
+    // it ensures each window will have its selected tab loaded.
+    if (willRestoreImmediately) {
+      this.restoreTabContent(tab, loadArguments, reloadInFreshProcess);
+    } else if (!forceOnDemand) {
+      this.restoreNextTab();
+    }
+
+    // Decrease the busy state counter after we're done.
+    this._setWindowStateReady(window);
+  },
+
+  /**
+   * Kicks off restoring the given tab.
+   *
+   * @param aTab
+   *        the tab to restore
+   * @param aLoadArguments
+   *        optional load arguments used for loadURI()
+   * @param aReloadInFreshProcess
+   *        true if we want to reload into a fresh process
+   */
+  restoreTabContent: function (aTab, aLoadArguments = null, aReloadInFreshProcess = false) {
+    if (aTab.hasAttribute("customizemode") && !aLoadArguments) {
+      return;
+    }
+
+    let browser = aTab.linkedBrowser;
+    let window = aTab.ownerGlobal;
+    let tabbrowser = window.gBrowser;
+    let tabData = TabState.clone(aTab);
+    let activeIndex = tabData.index - 1;
+    let activePageData = tabData.entries[activeIndex] || null;
+    let uri = activePageData ? activePageData.url || null : null;
+    if (aLoadArguments) {
+      uri = aLoadArguments.uri;
+      if (aLoadArguments.userContextId) {
+        browser.setAttribute("usercontextid", aLoadArguments.userContextId);
+      }
+    }
+
+    // We have to mark this tab as restoring first, otherwise
+    // the "pending" attribute will be applied to the linked
+    // browser, which removes it from the display list. We cannot
+    // flip the remoteness of any browser that is not being displayed.
+    this.markTabAsRestoring(aTab);
+
+    // We need a new frameloader either if we are reloading into a fresh
+    // process, or we have a browser with a grouped session history (as we don't
+    // support restoring into browsers with grouped session histories directly).
+    let newFrameloader =
+      aReloadInFreshProcess || !!browser.frameLoader.groupedSHistory;
+    let isRemotenessUpdate =
+      tabbrowser.updateBrowserRemotenessByURL(browser, uri, {
+        freshProcess: aReloadInFreshProcess,
+        newFrameloader: newFrameloader,
+      });
+
+    if (isRemotenessUpdate) {
+      // We updated the remoteness, so we need to send the history down again.
+      //
+      // Start a new epoch to discard all frame script messages relating to a
+      // previous epoch. All async messages that are still on their way to chrome
+      // will be ignored and don't override any tab data set when restoring.
+      let epoch = this.startNextEpoch(browser);
+
+      browser.messageManager.sendAsyncMessage("SessionStore:restoreHistory", {
+        tabData: tabData,
+        epoch: epoch,
+        loadArguments: aLoadArguments,
+        isRemotenessUpdate,
+      });
+
+    }
+
+    // If the restored browser wants to show view source content, start up a
+    // view source browser that will load the required frame script.
+    if (uri && ViewSourceBrowser.isViewSource(uri)) {
+      new ViewSourceBrowser(browser);
+    }
+
+    browser.messageManager.sendAsyncMessage("SessionStore:restoreTabContent",
+      {loadArguments: aLoadArguments, isRemotenessUpdate});
+  },
+
+  /**
+   * Marks a given pending tab as restoring.
+   *
+   * @param aTab
+   *        the pending tab to mark as restoring
+   */
+  markTabAsRestoring(aTab) {
+    let browser = aTab.linkedBrowser;
+    if (browser.__SS_restoreState != TAB_STATE_NEEDS_RESTORE) {
+      throw new Error("Given tab is not pending.");
+    }
+
+    // Make sure that this tab is removed from the priority queue.
+    TabRestoreQueue.remove(aTab);
+
+    // Increase our internal count.
+    this._tabsRestoringCount++;
+
+    // Set this tab's state to restoring
+    browser.__SS_restoreState = TAB_STATE_RESTORING;
+    browser.removeAttribute("pending");
+    aTab.removeAttribute("pending");
+  },
+
+  /**
+   * This _attempts_ to restore the next available tab. If the restore fails,
+   * then we will attempt the next one.
+   * There are conditions where this won't do anything:
+   *   if we're in the process of quitting
+   *   if there are no tabs to restore
+   *   if we have already reached the limit for number of tabs to restore
+   */
+  restoreNextTab: function ssi_restoreNextTab() {
+    // If we call in here while quitting, we don't actually want to do anything
+    if (RunState.isQuitting)
+      return;
+
+    // Don't exceed the maximum number of concurrent tab restores.
+    if (this._tabsRestoringCount >= MAX_CONCURRENT_TAB_RESTORES)
+      return;
+
+    let tab = TabRestoreQueue.shift();
+    if (tab) {
+      this.restoreTabContent(tab);
+    }
+  },
+
+  /**
+   * Restore visibility and dimension features to a window
+   * @param aWindow
+   *        Window reference
+   * @param aWinData
+   *        Object containing session data for the window
+   */
+  restoreWindowFeatures: function ssi_restoreWindowFeatures(aWindow, aWinData) {
+    var hidden = (aWinData.hidden)?aWinData.hidden.split(","):[];
+    WINDOW_HIDEABLE_FEATURES.forEach(function(aItem) {
+      aWindow[aItem].visible = hidden.indexOf(aItem) == -1;
+    });
+
+    if (aWinData.isPopup) {
+      this._windows[aWindow.__SSi].isPopup = true;
+      if (aWindow.gURLBar) {
+        aWindow.gURLBar.readOnly = true;
+        aWindow.gURLBar.setAttribute("enablehistory", "false");
+      }
+    }
+    else {
+      delete this._windows[aWindow.__SSi].isPopup;
+      if (aWindow.gURLBar) {
+        aWindow.gURLBar.readOnly = false;
+        aWindow.gURLBar.setAttribute("enablehistory", "true");
+      }
+    }
+
+    var _this = this;
+    aWindow.setTimeout(function() {
+      _this.restoreDimensions.apply(_this, [aWindow,
+        +(aWinData.width || 0),
+        +(aWinData.height || 0),
+        "screenX" in aWinData ? +aWinData.screenX : NaN,
+        "screenY" in aWinData ? +aWinData.screenY : NaN,
+        aWinData.sizemode || "", aWinData.sidebar || ""]);
+    }, 0);
+  },
+
+  /**
+   * Restore a window's dimensions
+   * @param aWidth
+   *        Window width
+   * @param aHeight
+   *        Window height
+   * @param aLeft
+   *        Window left
+   * @param aTop
+   *        Window top
+   * @param aSizeMode
+   *        Window size mode (eg: maximized)
+   * @param aSidebar
+   *        Sidebar command
+   */
+  restoreDimensions: function ssi_restoreDimensions(aWindow, aWidth, aHeight, aLeft, aTop, aSizeMode, aSidebar) {
+    var win = aWindow;
+    var _this = this;
+    function win_(aName) { return _this._getWindowDimension(win, aName); }
+
+    // find available space on the screen where this window is being placed
+    let screen = gScreenManager.screenForRect(aLeft, aTop, aWidth, aHeight);
+    if (screen) {
+      let screenLeft = {}, screenTop = {}, screenWidth = {}, screenHeight = {};
+      screen.GetAvailRectDisplayPix(screenLeft, screenTop, screenWidth, screenHeight);
+      // screenX/Y are based on the origin of the screen's desktop-pixel coordinate space
+      let screenLeftCss = screenLeft.value;
+      let screenTopCss = screenTop.value;
+      // convert screen's device pixel dimensions to CSS px dimensions
+      screen.GetAvailRect(screenLeft, screenTop, screenWidth, screenHeight);
+      let cssToDevScale = screen.defaultCSSScaleFactor;
+      let screenRightCss = screenLeftCss + screenWidth.value / cssToDevScale;
+      let screenBottomCss = screenTopCss + screenHeight.value / cssToDevScale;
+
+      // Pull the window within the screen's bounds (allowing a little slop
+      // for windows that may be deliberately placed with their border off-screen
+      // as when Win10 "snaps" a window to the left/right edge -- bug 1276516).
+      // First, ensure the left edge is large enough...
+      if (aLeft < screenLeftCss - SCREEN_EDGE_SLOP) {
+        aLeft = screenLeftCss;
+      }
+      // Then check the resulting right edge, and reduce it if necessary.
+      let right = aLeft + aWidth;
+      if (right > screenRightCss + SCREEN_EDGE_SLOP) {
+        right = screenRightCss;
+        // See if we can move the left edge leftwards to maintain width.
+        if (aLeft > screenLeftCss) {
+          aLeft = Math.max(right - aWidth, screenLeftCss);
+        }
+      }
+      // Finally, update aWidth to account for the adjusted left and right edges.
+      aWidth = right - aLeft;
+
+      // And do the same in the vertical dimension.
+      if (aTop < screenTopCss - SCREEN_EDGE_SLOP) {
+        aTop = screenTopCss;
+      }
+      let bottom = aTop + aHeight;
+      if (bottom > screenBottomCss + SCREEN_EDGE_SLOP) {
+        bottom = screenBottomCss;
+        if (aTop > screenTopCss) {
+          aTop = Math.max(bottom - aHeight, screenTopCss);
+        }
+      }
+      aHeight = bottom - aTop;
+    }
+
+    // only modify those aspects which aren't correct yet
+    if (!isNaN(aLeft) && !isNaN(aTop) && (aLeft != win_("screenX") || aTop != win_("screenY"))) {
+      aWindow.moveTo(aLeft, aTop);
+    }
+    if (aWidth && aHeight && (aWidth != win_("width") || aHeight != win_("height"))) {
+      // Don't resize the window if it's currently maximized and we would
+      // maximize it again shortly after.
+      if (aSizeMode != "maximized" || win_("sizemode") != "maximized") {
+        aWindow.resizeTo(aWidth, aHeight);
+      }
+    }
+    if (aSizeMode && win_("sizemode") != aSizeMode)
+    {
+      switch (aSizeMode)
+      {
+      case "maximized":
+        aWindow.maximize();
+        break;
+      case "minimized":
+        aWindow.minimize();
+        break;
+      case "normal":
+        aWindow.restore();
+        break;
+      }
+    }
+    var sidebar = aWindow.document.getElementById("sidebar-box");
+    if (sidebar.getAttribute("sidebarcommand") != aSidebar) {
+      aWindow.SidebarUI.show(aSidebar);
+    }
+    // since resizing/moving a window brings it to the foreground,
+    // we might want to re-focus the last focused window
+    if (this.windowToFocus) {
+      this.windowToFocus.focus();
+    }
+  },
+
+  /* ........ Disk Access .............. */
+
+  /**
+   * Save the current session state to disk, after a delay.
+   *
+   * @param aWindow (optional)
+   *        Will mark the given window as dirty so that we will recollect its
+   *        data before we start writing.
+   */
+  saveStateDelayed: function (aWindow = null) {
+    if (aWindow) {
+      DirtyWindows.add(aWindow);
+    }
+
+    SessionSaver.runDelayed();
+  },
+
+  /* ........ Auxiliary Functions .............. */
+
+  /**
+   * Remove a closed window from the list of closed windows and indicate that
+   * the change should be notified.
+   *
+   * @param index
+   *        The index of the window in this._closedWindows.
+   *
+   * @returns Array of closed windows.
+   */
+  _removeClosedWindow(index) {
+    let windows = this._closedWindows.splice(index, 1);
+    this._closedObjectsChanged = true;
+    return windows;
+  },
+
+  /**
+   * Notifies observers that the list of closed tabs and/or windows has changed.
+   * Waits a tick to allow SessionStorage a chance to register the change.
+   */
+  _notifyOfClosedObjectsChange() {
+    if (!this._closedObjectsChanged) {
+      return;
+    }
+    this._closedObjectsChanged = false;
+    setTimeout(() => {
+      Services.obs.notifyObservers(null, NOTIFY_CLOSED_OBJECTS_CHANGED, null);
+    }, 0);
+  },
+
+  /**
+   * Determines whether or not a tab that is being restored needs
+   * to have its remoteness flipped first.
+   *
+   * @param (object) with the following properties:
+   *
+   *        tabbrowser (<xul:tabbrowser>):
+   *          The tabbrowser that the browser belongs to.
+   *
+   *        tab (<xul:tab>):
+   *          The tab being restored
+   *
+   *        willRestoreImmediately (bool):
+   *          true if the tab is going to have its content
+   *          restored immediately by the caller.
+   *
+   */
+  _maybeUpdateBrowserRemoteness({ tabbrowser, tab,
+                                  willRestoreImmediately }) {
+    // If the browser we're attempting to restore happens to be
+    // remote, we need to flip it back to non-remote if it's going
+    // to go into the pending background tab state. This is to make
+    // sure that a background tab can't crash if it hasn't yet
+    // been restored.
+    //
+    // Normally, when a window is restored, the tabs that SessionStore
+    // inserts are non-remote - but the initial browser is, by default,
+    // remote, so this check and flip covers this case. The other case
+    // is when window state is overwriting the state of an existing
+    // window with some remote tabs.
+    let browser = tab.linkedBrowser;
+
+    // There are two ways that a tab might start restoring its content
+    // very soon - either the caller is going to restore the content
+    // immediately, or the TabRestoreQueue is set up so that the tab
+    // content is going to be restored in the very near future. In
+    // either case, we don't want to flip remoteness, since the browser
+    // will soon be loading content.
+    let willRestore = willRestoreImmediately ||
+                      TabRestoreQueue.willRestoreSoon(tab);
+
+    if (browser.isRemoteBrowser && !willRestore) {
+      tabbrowser.updateBrowserRemoteness(browser, false);
+    }
+  },
+
+  /**
+   * Update the session start time and send a telemetry measurement
+   * for the number of days elapsed since the session was started.
+   *
+   * @param state
+   *        The session state.
+   */
+  _updateSessionStartTime: function ssi_updateSessionStartTime(state) {
+    // Attempt to load the session start time from the session state
+    if (state.session && state.session.startTime) {
+      this._sessionStartTime = state.session.startTime;
+    }
+  },
+
+  /**
+   * call a callback for all currently opened browser windows
+   * (might miss the most recent one)
+   * @param aFunc
+   *        Callback each window is passed to
+   */
+  _forEachBrowserWindow: function ssi_forEachBrowserWindow(aFunc) {
+    var windowsEnum = Services.wm.getEnumerator("navigator:browser");
+
+    while (windowsEnum.hasMoreElements()) {
+      var window = windowsEnum.getNext();
+      if (window.__SSi && !window.closed) {
+        aFunc.call(this, window);
+      }
+    }
+  },
+
+  /**
+   * Returns most recent window
+   * @returns Window reference
+   */
+  _getMostRecentBrowserWindow: function ssi_getMostRecentBrowserWindow() {
+    return RecentWindow.getMostRecentBrowserWindow({ allowPopups: true });
+  },
+
+  /**
+   * Calls onClose for windows that are determined to be closed but aren't
+   * destroyed yet, which would otherwise cause getBrowserState and
+   * setBrowserState to treat them as open windows.
+   */
+  _handleClosedWindows: function ssi_handleClosedWindows() {
+    var windowsEnum = Services.wm.getEnumerator("navigator:browser");
+
+    let promises = [];
+    while (windowsEnum.hasMoreElements()) {
+      var window = windowsEnum.getNext();
+      if (window.closed) {
+        promises.push(this.onClose(window));
+      }
+    }
+    return Promise.all(promises);
+  },
+
+  /**
+   * open a new browser window for a given session state
+   * called when restoring a multi-window session
+   * @param aState
+   *        Object containing session data
+   */
+  _openWindowWithState: function ssi_openWindowWithState(aState) {
+    var argString = Cc["@mozilla.org/supports-string;1"].
+                    createInstance(Ci.nsISupportsString);
+    argString.data = "";
+
+    // Build feature string
+    let features = "chrome,dialog=no,macsuppressanimation,all";
+    let winState = aState.windows[0];
+    WINDOW_ATTRIBUTES.forEach(function(aFeature) {
+      // Use !isNaN as an easy way to ignore sizemode and check for numbers
+      if (aFeature in winState && !isNaN(winState[aFeature]))
+        features += "," + aFeature + "=" + winState[aFeature];
+    });
+
+    if (winState.isPrivate) {
+      features += ",private";
+    }
+
+    var window =
+      Services.ww.openWindow(null, this._prefBranch.getCharPref("chromeURL"),
+                             "_blank", features, argString);
+
+    do {
+      var ID = "window" + Math.random();
+    } while (ID in this._statesToRestore);
+    this._statesToRestore[(window.__SS_restoreID = ID)] = aState;
+
+    return window;
+  },
+
+  /**
+   * Whether or not to resume session, if not recovering from a crash.
+   * @returns bool
+   */
+  _doResumeSession: function ssi_doResumeSession() {
+    return this._prefBranch.getIntPref("startup.page") == 3 ||
+           this._prefBranch.getBoolPref("sessionstore.resume_session_once");
+  },
+
+  /**
+   * whether the user wants to load any other page at startup
+   * (except the homepage) - needed for determining whether to overwrite the current tabs
+   * C.f.: nsBrowserContentHandler's defaultArgs implementation.
+   * @returns bool
+   */
+  _isCmdLineEmpty: function ssi_isCmdLineEmpty(aWindow, aState) {
+    var pinnedOnly = aState.windows &&
+                     aState.windows.every(win =>
+                       win.tabs.every(tab => tab.pinned));
+
+    let hasFirstArgument = aWindow.arguments && aWindow.arguments[0];
+    if (!pinnedOnly) {
+      let defaultArgs = Cc["@mozilla.org/browser/clh;1"].
+                        getService(Ci.nsIBrowserHandler).defaultArgs;
+      if (aWindow.arguments &&
+          aWindow.arguments[0] &&
+          aWindow.arguments[0] == defaultArgs)
+        hasFirstArgument = false;
+    }
+
+    return !hasFirstArgument;
+  },
+
+  /**
+   * on popup windows, the XULWindow's attributes seem not to be set correctly
+   * we use thus JSDOMWindow attributes for sizemode and normal window attributes
+   * (and hope for reasonable values when maximized/minimized - since then
+   * outerWidth/outerHeight aren't the dimensions of the restored window)
+   * @param aWindow
+   *        Window reference
+   * @param aAttribute
+   *        String sizemode | width | height | other window attribute
+   * @returns string
+   */
+  _getWindowDimension: function ssi_getWindowDimension(aWindow, aAttribute) {
+    if (aAttribute == "sizemode") {
+      switch (aWindow.windowState) {
+      case aWindow.STATE_FULLSCREEN:
+      case aWindow.STATE_MAXIMIZED:
+        return "maximized";
+      case aWindow.STATE_MINIMIZED:
+        return "minimized";
+      default:
+        return "normal";
+      }
+    }
+
+    var dimension;
+    switch (aAttribute) {
+    case "width":
+      dimension = aWindow.outerWidth;
+      break;
+    case "height":
+      dimension = aWindow.outerHeight;
+      break;
+    default:
+      dimension = aAttribute in aWindow ? aWindow[aAttribute] : "";
+      break;
+    }
+
+    if (aWindow.windowState == aWindow.STATE_NORMAL) {
+      return dimension;
+    }
+    return aWindow.document.documentElement.getAttribute(aAttribute) || dimension;
+  },
+
+  /**
+   * @param aState is a session state
+   * @param aRecentCrashes is the number of consecutive crashes
+   * @returns whether a restore page will be needed for the session state
+   */
+  _needsRestorePage: function ssi_needsRestorePage(aState, aRecentCrashes) {
+    const SIX_HOURS_IN_MS = 6 * 60 * 60 * 1000;
+
+    // don't display the page when there's nothing to restore
+    let winData = aState.windows || null;
+    if (!winData || winData.length == 0)
+      return false;
+
+    // don't wrap a single about:sessionrestore page
+    if (this._hasSingleTabWithURL(winData, "about:sessionrestore") ||
+        this._hasSingleTabWithURL(winData, "about:welcomeback")) {
+      return false;
+    }
+
+    // don't automatically restore in Safe Mode
+    if (Services.appinfo.inSafeMode)
+      return true;
+
+    let max_resumed_crashes =
+      this._prefBranch.getIntPref("sessionstore.max_resumed_crashes");
+    let sessionAge = aState.session && aState.session.lastUpdate &&
+                     (Date.now() - aState.session.lastUpdate);
+
+    return max_resumed_crashes != -1 &&
+           (aRecentCrashes > max_resumed_crashes ||
+            sessionAge && sessionAge >= SIX_HOURS_IN_MS);
+  },
+
+  /**
+   * @param aWinData is the set of windows in session state
+   * @param aURL is the single URL we're looking for
+   * @returns whether the window data contains only the single URL passed
+   */
+  _hasSingleTabWithURL: function(aWinData, aURL) {
+    if (aWinData &&
+        aWinData.length == 1 &&
+        aWinData[0].tabs &&
+        aWinData[0].tabs.length == 1 &&
+        aWinData[0].tabs[0].entries &&
+        aWinData[0].tabs[0].entries.length == 1) {
+      return aURL == aWinData[0].tabs[0].entries[0].url;
+    }
+    return false;
+  },
+
+  /**
+   * Determine if the tab state we're passed is something we should save. This
+   * is used when closing a tab or closing a window with a single tab
+   *
+   * @param aTabState
+   *        The current tab state
+   * @returns boolean
+   */
+  _shouldSaveTabState: function ssi_shouldSaveTabState(aTabState) {
+    // If the tab has only a transient about: history entry, no other
+    // session history, and no userTypedValue, then we don't actually want to
+    // store this tab's data.
+    return aTabState.entries.length &&
+           !(aTabState.entries.length == 1 &&
+                (aTabState.entries[0].url == "about:blank" ||
+                 aTabState.entries[0].url == "about:newtab" ||
+                 aTabState.entries[0].url == "about:printpreview" ||
+                 aTabState.entries[0].url == "about:privatebrowsing") &&
+                 !aTabState.userTypedValue);
+  },
+
+  /**
+   * Determine if the tab state we're passed is something we should keep to be
+   * reopened at session restore. This is used when we are saving the current
+   * session state to disk. This method is very similar to _shouldSaveTabState,
+   * however, "about:blank" and "about:newtab" tabs will still be saved to disk.
+   *
+   * @param aTabState
+   *        The current tab state
+   * @returns boolean
+   */
+  _shouldSaveTab: function ssi_shouldSaveTab(aTabState) {
+    // If the tab has one of the following transient about: history entry,
+    // then we don't actually want to write this tab's data to disk.
+    return aTabState.userTypedValue ||
+           (aTabState.entries.length &&
+            !(aTabState.entries[0].url == "about:printpreview" ||
+              aTabState.entries[0].url == "about:privatebrowsing"));
+  },
+
+  /**
+   * This is going to take a state as provided at startup (via
+   * nsISessionStartup.state) and split it into 2 parts. The first part
+   * (defaultState) will be a state that should still be restored at startup,
+   * while the second part (state) is a state that should be saved for later.
+   * defaultState will be comprised of windows with only pinned tabs, extracted
+   * from state. It will contain the cookies that go along with the history
+   * entries in those tabs. It will also contain window position information.
+   *
+   * defaultState will be restored at startup. state will be passed into
+   * LastSession and will be kept in case the user explicitly wants
+   * to restore the previous session (publicly exposed as restoreLastSession).
+   *
+   * @param state
+   *        The state, presumably from nsISessionStartup.state
+   * @returns [defaultState, state]
+   */
+  _prepDataForDeferredRestore: function ssi_prepDataForDeferredRestore(state) {
+    // Make sure that we don't modify the global state as provided by
+    // nsSessionStartup.state.
+    state = Cu.cloneInto(state, {});
+
+    let defaultState = { windows: [], selectedWindow: 1 };
+
+    state.selectedWindow = state.selectedWindow || 1;
+
+    // Look at each window, remove pinned tabs, adjust selectedindex,
+    // remove window if necessary.
+    for (let wIndex = 0; wIndex < state.windows.length;) {
+      let window = state.windows[wIndex];
+      window.selected = window.selected || 1;
+      // We're going to put the state of the window into this object
+      let pinnedWindowState = { tabs: [], cookies: []};
+      for (let tIndex = 0; tIndex < window.tabs.length;) {
+        if (window.tabs[tIndex].pinned) {
+          // Adjust window.selected
+          if (tIndex + 1 < window.selected)
+            window.selected -= 1;
+          else if (tIndex + 1 == window.selected)
+            pinnedWindowState.selected = pinnedWindowState.tabs.length + 2;
+            // + 2 because the tab isn't actually in the array yet
+
+          // Now add the pinned tab to our window
+          pinnedWindowState.tabs =
+            pinnedWindowState.tabs.concat(window.tabs.splice(tIndex, 1));
+          // We don't want to increment tIndex here.
+          continue;
+        }
+        tIndex++;
+      }
+
+      // At this point the window in the state object has been modified (or not)
+      // We want to build the rest of this new window object if we have pinnedTabs.
+      if (pinnedWindowState.tabs.length) {
+        // First get the other attributes off the window
+        WINDOW_ATTRIBUTES.forEach(function(attr) {
+          if (attr in window) {
+            pinnedWindowState[attr] = window[attr];
+            delete window[attr];
+          }
+        });
+        // We're just copying position data into the pinned window.
+        // Not copying over:
+        // - _closedTabs
+        // - extData
+        // - isPopup
+        // - hidden
+
+        // Assign a unique ID to correlate the window to be opened with the
+        // remaining data
+        window.__lastSessionWindowID = pinnedWindowState.__lastSessionWindowID
+                                     = "" + Date.now() + Math.random();
+
+        // Extract the cookies that belong with each pinned tab
+        this._splitCookiesFromWindow(window, pinnedWindowState);
+
+        // Actually add this window to our defaultState
+        defaultState.windows.push(pinnedWindowState);
+        // Remove the window from the state if it doesn't have any tabs
+        if (!window.tabs.length) {
+          if (wIndex + 1 <= state.selectedWindow)
+            state.selectedWindow -= 1;
+          else if (wIndex + 1 == state.selectedWindow)
+            defaultState.selectedIndex = defaultState.windows.length + 1;
+
+          state.windows.splice(wIndex, 1);
+          // We don't want to increment wIndex here.
+          continue;
+        }
+
+
+      }
+      wIndex++;
+    }
+
+    return [defaultState, state];
+  },
+
+  /**
+   * Splits out the cookies from aWinState into aTargetWinState based on the
+   * tabs that are in aTargetWinState.
+   * This alters the state of aWinState and aTargetWinState.
+   */
+  _splitCookiesFromWindow:
+    function ssi_splitCookiesFromWindow(aWinState, aTargetWinState) {
+    if (!aWinState.cookies || !aWinState.cookies.length)
+      return;
+
+    // Get the hosts for history entries in aTargetWinState
+    let cookieHosts = SessionCookies.getHostsForWindow(aTargetWinState);
+
+    // By creating a regex we reduce overhead and there is only one loop pass
+    // through either array (cookieHosts and aWinState.cookies).
+    let hosts = Object.keys(cookieHosts).join("|").replace(/\./g, "\\.");
+    // If we don't actually have any hosts, then we don't want to do anything.
+    if (!hosts.length)
+      return;
+    let cookieRegex = new RegExp(".*(" + hosts + ")");
+    for (let cIndex = 0; cIndex < aWinState.cookies.length;) {
+      if (cookieRegex.test(aWinState.cookies[cIndex].host)) {
+        aTargetWinState.cookies =
+          aTargetWinState.cookies.concat(aWinState.cookies.splice(cIndex, 1));
+        continue;
+      }
+      cIndex++;
+    }
+  },
+
+  _sendRestoreCompletedNotifications: function ssi_sendRestoreCompletedNotifications() {
+    // not all windows restored, yet
+    if (this._restoreCount > 1) {
+      this._restoreCount--;
+      return;
+    }
+
+    // observers were already notified
+    if (this._restoreCount == -1)
+      return;
+
+    // This was the last window restored at startup, notify observers.
+    Services.obs.notifyObservers(null,
+      this._browserSetState ? NOTIFY_BROWSER_STATE_RESTORED : NOTIFY_WINDOWS_RESTORED,
+      "");
+
+    this._browserSetState = false;
+    this._restoreCount = -1;
+  },
+
+   /**
+   * Set the given window's busy state
+   * @param aWindow the window
+   * @param aValue the window's busy state
+   */
+  _setWindowStateBusyValue:
+    function ssi_changeWindowStateBusyValue(aWindow, aValue) {
+
+    this._windows[aWindow.__SSi].busy = aValue;
+
+    // Keep the to-be-restored state in sync because that is returned by
+    // getWindowState() as long as the window isn't loaded, yet.
+    if (!this._isWindowLoaded(aWindow)) {
+      let stateToRestore = this._statesToRestore[aWindow.__SS_restoreID].windows[0];
+      stateToRestore.busy = aValue;
+    }
+  },
+
+  /**
+   * Set the given window's state to 'not busy'.
+   * @param aWindow the window
+   */
+  _setWindowStateReady: function ssi_setWindowStateReady(aWindow) {
+    let newCount = (this._windowBusyStates.get(aWindow) || 0) - 1;
+    if (newCount < 0) {
+      throw new Error("Invalid window busy state (less than zero).");
+    }
+    this._windowBusyStates.set(aWindow, newCount);
+
+    if (newCount == 0) {
+      this._setWindowStateBusyValue(aWindow, false);
+      this._sendWindowStateEvent(aWindow, "Ready");
+    }
+  },
+
+  /**
+   * Set the given window's state to 'busy'.
+   * @param aWindow the window
+   */
+  _setWindowStateBusy: function ssi_setWindowStateBusy(aWindow) {
+    let newCount = (this._windowBusyStates.get(aWindow) || 0) + 1;
+    this._windowBusyStates.set(aWindow, newCount);
+
+    if (newCount == 1) {
+      this._setWindowStateBusyValue(aWindow, true);
+      this._sendWindowStateEvent(aWindow, "Busy");
+    }
+  },
+
+  /**
+   * Dispatch an SSWindowState_____ event for the given window.
+   * @param aWindow the window
+   * @param aType the type of event, SSWindowState will be prepended to this string
+   */
+  _sendWindowStateEvent: function ssi_sendWindowStateEvent(aWindow, aType) {
+    let event = aWindow.document.createEvent("Events");
+    event.initEvent("SSWindowState" + aType, true, false);
+    aWindow.dispatchEvent(event);
+  },
+
+  /**
+   * Dispatch the SSWindowRestored event for the given window.
+   * @param aWindow
+   *        The window which has been restored
+   */
+  _sendWindowRestoredNotification(aWindow) {
+    let event = aWindow.document.createEvent("Events");
+    event.initEvent("SSWindowRestored", true, false);
+    aWindow.dispatchEvent(event);
+  },
+
+  /**
+   * Dispatch the SSTabRestored event for the given tab.
+   * @param aTab
+   *        The tab which has been restored
+   * @param aIsRemotenessUpdate
+   *        True if this tab was restored due to flip from running from
+   *        out-of-main-process to in-main-process or vice-versa.
+   */
+  _sendTabRestoredNotification(aTab, aIsRemotenessUpdate) {
+    let event = aTab.ownerDocument.createEvent("CustomEvent");
+    event.initCustomEvent("SSTabRestored", true, false, {
+      isRemotenessUpdate: aIsRemotenessUpdate,
+    });
+    aTab.dispatchEvent(event);
+  },
+
+  /**
+   * @param aWindow
+   *        Window reference
+   * @returns whether this window's data is still cached in _statesToRestore
+   *          because it's not fully loaded yet
+   */
+  _isWindowLoaded: function ssi_isWindowLoaded(aWindow) {
+    return !aWindow.__SS_restoreID;
+  },
+
+  /**
+   * Replace "Loading..." with the tab label (with minimal side-effects)
+   * @param aString is the string the title is stored in
+   * @param aTabbrowser is a tabbrowser object, containing aTab
+   * @param aTab is the tab whose title we're updating & using
+   *
+   * @returns aString that has been updated with the new title
+   */
+  _replaceLoadingTitle : function ssi_replaceLoadingTitle(aString, aTabbrowser, aTab) {
+    if (aString == aTabbrowser.mStringBundle.getString("tabs.connecting")) {
+      aTabbrowser.setTabTitle(aTab);
+      [aString, aTab.label] = [aTab.label, aString];
+    }
+    return aString;
+  },
+
+  /**
+   * Resize this._closedWindows to the value of the pref, except in the case
+   * where we don't have any non-popup windows on Windows and Linux. Then we must
+   * resize such that we have at least one non-popup window.
+   */
+  _capClosedWindows : function ssi_capClosedWindows() {
+    if (this._closedWindows.length <= this._max_windows_undo)
+      return;
+    let spliceTo = this._max_windows_undo;
+#ifndef XP_MACOSX
+    let normalWindowIndex = 0;
+    // try to find a non-popup window in this._closedWindows
+    while (normalWindowIndex < this._closedWindows.length &&
+           !!this._closedWindows[normalWindowIndex].isPopup)
+      normalWindowIndex++;
+    if (normalWindowIndex >= this._max_windows_undo)
+      spliceTo = normalWindowIndex + 1;
+#endif
+    if (spliceTo < this._closedWindows.length) {
+      this._closedWindows.splice(spliceTo, this._closedWindows.length);
+      this._closedObjectsChanged = true;
+    }
+  },
+
+  /**
+   * Clears the set of windows that are "resurrected" before writing to disk to
+   * make closing windows one after the other until shutdown work as expected.
+   *
+   * This function should only be called when we are sure that there has been
+   * a user action that indicates the browser is actively being used and all
+   * windows that have been closed before are not part of a series of closing
+   * windows.
+   */
+  _clearRestoringWindows: function ssi_clearRestoringWindows() {
+    for (let i = 0; i < this._closedWindows.length; i++) {
+      delete this._closedWindows[i]._shouldRestore;
+    }
+  },
+
+  /**
+   * Reset state to prepare for a new session state to be restored.
+   */
+  _resetRestoringState: function ssi_initRestoringState() {
+    TabRestoreQueue.reset();
+    this._tabsRestoringCount = 0;
+  },
+
+  /**
+   * Reset the restoring state for a particular tab. This will be called when
+   * removing a tab or when a tab needs to be reset (it's being overwritten).
+   *
+   * @param aTab
+   *        The tab that will be "reset"
+   */
+  _resetLocalTabRestoringState: function (aTab) {
+    NS_ASSERT(aTab.linkedBrowser.__SS_restoreState,
+              "given tab is not restoring");
+
+    let browser = aTab.linkedBrowser;
+
+    // Keep the tab's previous state for later in this method
+    let previousState = browser.__SS_restoreState;
+
+    // The browser is no longer in any sort of restoring state.
+    delete browser.__SS_restoreState;
+
+    aTab.removeAttribute("pending");
+    browser.removeAttribute("pending");
+
+    if (previousState == TAB_STATE_RESTORING) {
+      if (this._tabsRestoringCount)
+        this._tabsRestoringCount--;
+    } else if (previousState == TAB_STATE_NEEDS_RESTORE) {
+      // Make sure that the tab is removed from the list of tabs to restore.
+      // Again, this is normally done in restoreTabContent, but that isn't being called
+      // for this tab.
+      TabRestoreQueue.remove(aTab);
+    }
+  },
+
+  _resetTabRestoringState: function (tab) {
+    NS_ASSERT(tab.linkedBrowser.__SS_restoreState,
+              "given tab is not restoring");
+
+    let browser = tab.linkedBrowser;
+    browser.messageManager.sendAsyncMessage("SessionStore:resetRestore", {});
+    this._resetLocalTabRestoringState(tab);
+  },
+
+  /**
+   * Each fresh tab starts out with epoch=0. This function can be used to
+   * start a next epoch by incrementing the current value. It will enables us
+   * to ignore stale messages sent from previous epochs. The function returns
+   * the new epoch ID for the given |browser|.
+   */
+  startNextEpoch(browser) {
+    let next = this.getCurrentEpoch(browser) + 1;
+    this._browserEpochs.set(browser.permanentKey, next);
+    return next;
+  },
+
+  /**
+   * Manually set the epoch to a given value.
+   */
+  setCurrentEpoch(aBrowser, aEpoch) {
+    this._browserEpochs.set(aBrowser.permanentKey, aEpoch);
+    return aEpoch;
+  },
+
+  /**
+   * Returns the current epoch for the given <browser>. If we haven't assigned
+   * a new epoch this will default to zero for new tabs.
+   */
+  getCurrentEpoch(browser) {
+    return this._browserEpochs.get(browser.permanentKey) || 0;
+  },
+
+  /**
+   * Each time a <browser> element is restored, we increment its "epoch". To
+   * check if a message from content-sessionStore.js is out of date, we can
+   * compare the epoch received with the message to the <browser> element's
+   * epoch. This function does that, and returns true if |epoch| is up-to-date
+   * with respect to |browser|.
+   */
+  isCurrentEpoch: function (browser, epoch) {
+    return this.getCurrentEpoch(browser) == epoch;
+  },
+
+  /**
+   * Resets the epoch for a given <browser>. We need to this every time we
+   * receive a hint that a new docShell has been loaded into the browser as
+   * the frame script starts out with epoch=0.
+   */
+  resetEpoch(browser) {
+    this._browserEpochs.delete(browser.permanentKey);
+  },
+
+  /**
+   * Handle an error report from a content process.
+   */
+  reportInternalError(data) {
+    // For the moment, we only report errors through Telemetry.
+    if (data.telemetry) {
+      for (let key of Object.keys(data.telemetry)) {
+        let histogram = Telemetry.getHistogramById(key);
+        histogram.add(data.telemetry[key]);
+      }
+    }
+  },
+
+  /**
+   * Countdown for a given duration, skipping beats if the computer is too busy,
+   * sleeping or otherwise unavailable.
+   *
+   * @param {number} delay An approximate delay to wait in milliseconds (rounded
+   * up to the closest second).
+   *
+   * @return Promise
+   */
+  looseTimer(delay) {
+    let DELAY_BEAT = 1000;
+    let timer = Cc["@mozilla.org/timer;1"].createInstance(Ci.nsITimer);
+    let beats = Math.ceil(delay / DELAY_BEAT);
+    let promise =  new Promise(resolve => {
+      timer.initWithCallback(function() {
+        if (beats <= 0) {
+          resolve();
+        }
+        --beats;
+      }, DELAY_BEAT, Ci.nsITimer.TYPE_REPEATING_PRECISE_CAN_SKIP);
+    });
+    // Ensure that the timer is both canceled once we are done with it
+    // and not garbage-collected until then.
+    promise.then(() => timer.cancel(), () => timer.cancel());
+    return promise;
+  }
+};
+
+/**
+ * Priority queue that keeps track of a list of tabs to restore and returns
+ * the tab we should restore next, based on priority rules. We decide between
+ * pinned, visible and hidden tabs in that and FIFO order. Hidden tabs are only
+ * restored with restore_hidden_tabs=true.
+ */
+var TabRestoreQueue = {
+  // The separate buckets used to store tabs.
+  tabs: {priority: [], visible: [], hidden: []},
+
+  // Preferences used by the TabRestoreQueue to determine which tabs
+  // are restored automatically and which tabs will be on-demand.
+  prefs: {
+    // Lazy getter that returns whether tabs are restored on demand.
+    get restoreOnDemand() {
+      let updateValue = () => {
+        let value = Services.prefs.getBoolPref(PREF);
+        let definition = {value: value, configurable: true};
+        Object.defineProperty(this, "restoreOnDemand", definition);
+        return value;
+      }
+
+      const PREF = "browser.sessionstore.restore_on_demand";
+      Services.prefs.addObserver(PREF, updateValue, false);
+      return updateValue();
+    },
+
+    // Lazy getter that returns whether pinned tabs are restored on demand.
+    get restorePinnedTabsOnDemand() {
+      let updateValue = () => {
+        let value = Services.prefs.getBoolPref(PREF);
+        let definition = {value: value, configurable: true};
+        Object.defineProperty(this, "restorePinnedTabsOnDemand", definition);
+        return value;
+      }
+
+      const PREF = "browser.sessionstore.restore_pinned_tabs_on_demand";
+      Services.prefs.addObserver(PREF, updateValue, false);
+      return updateValue();
+    },
+
+    // Lazy getter that returns whether we should restore hidden tabs.
+    get restoreHiddenTabs() {
+      let updateValue = () => {
+        let value = Services.prefs.getBoolPref(PREF);
+        let definition = {value: value, configurable: true};
+        Object.defineProperty(this, "restoreHiddenTabs", definition);
+        return value;
+      }
+
+      const PREF = "browser.sessionstore.restore_hidden_tabs";
+      Services.prefs.addObserver(PREF, updateValue, false);
+      return updateValue();
+    }
+  },
+
+  // Resets the queue and removes all tabs.
+  reset: function () {
+    this.tabs = {priority: [], visible: [], hidden: []};
+  },
+
+  // Adds a tab to the queue and determines its priority bucket.
+  add: function (tab) {
+    let {priority, hidden, visible} = this.tabs;
+
+    if (tab.pinned) {
+      priority.push(tab);
+    } else if (tab.hidden) {
+      hidden.push(tab);
+    } else {
+      visible.push(tab);
+    }
+  },
+
+  // Removes a given tab from the queue, if it's in there.
+  remove: function (tab) {
+    let {priority, hidden, visible} = this.tabs;
+
+    // We'll always check priority first since we don't
+    // have an indicator if a tab will be there or not.
+    let set = priority;
+    let index = set.indexOf(tab);
+
+    if (index == -1) {
+      set = tab.hidden ? hidden : visible;
+      index = set.indexOf(tab);
+    }
+
+    if (index > -1) {
+      set.splice(index, 1);
+    }
+  },
+
+  // Returns and removes the tab with the highest priority.
+  shift: function () {
+    let set;
+    let {priority, hidden, visible} = this.tabs;
+
+    let {restoreOnDemand, restorePinnedTabsOnDemand} = this.prefs;
+    let restorePinned = !(restoreOnDemand && restorePinnedTabsOnDemand);
+    if (restorePinned && priority.length) {
+      set = priority;
+    } else if (!restoreOnDemand) {
+      if (visible.length) {
+        set = visible;
+      } else if (this.prefs.restoreHiddenTabs && hidden.length) {
+        set = hidden;
+      }
+    }
+
+    return set && set.shift();
+  },
+
+  // Moves a given tab from the 'hidden' to the 'visible' bucket.
+  hiddenToVisible: function (tab) {
+    let {hidden, visible} = this.tabs;
+    let index = hidden.indexOf(tab);
+
+    if (index > -1) {
+      hidden.splice(index, 1);
+      visible.push(tab);
+    }
+  },
+
+  // Moves a given tab from the 'visible' to the 'hidden' bucket.
+  visibleToHidden: function (tab) {
+    let {visible, hidden} = this.tabs;
+    let index = visible.indexOf(tab);
+
+    if (index > -1) {
+      visible.splice(index, 1);
+      hidden.push(tab);
+    }
+  },
+
+  /**
+   * Returns true if the passed tab is in one of the sets that we're
+   * restoring content in automatically.
+   *
+   * @param tab (<xul:tab>)
+   *        The tab to check
+   * @returns bool
+   */
+  willRestoreSoon: function (tab) {
+    let { priority, hidden, visible } = this.tabs;
+    let { restoreOnDemand, restorePinnedTabsOnDemand,
+          restoreHiddenTabs } = this.prefs;
+    let restorePinned = !(restoreOnDemand && restorePinnedTabsOnDemand);
+    let candidateSet = [];
+
+    if (restorePinned && priority.length)
+      candidateSet.push(...priority);
+
+    if (!restoreOnDemand) {
+      if (visible.length)
+        candidateSet.push(...visible);
+
+      if (restoreHiddenTabs && hidden.length)
+        candidateSet.push(...hidden);
+    }
+
+    return candidateSet.indexOf(tab) > -1;
+  },
+};
+
+// A map storing a closed window's state data until it goes aways (is GC'ed).
+// This ensures that API clients can still read (but not write) states of
+// windows they still hold a reference to but we don't.
+var DyingWindowCache = {
+  _data: new WeakMap(),
+
+  has: function (window) {
+    return this._data.has(window);
+  },
+
+  get: function (window) {
+    return this._data.get(window);
+  },
+
+  set: function (window, data) {
+    this._data.set(window, data);
+  },
+
+  remove: function (window) {
+    this._data.delete(window);
+  }
+};
+
+// A weak set of dirty windows. We use it to determine which windows we need to
+// recollect data for when getCurrentState() is called.
+var DirtyWindows = {
+  _data: new WeakMap(),
+
+  has: function (window) {
+    return this._data.has(window);
+  },
+
+  add: function (window) {
+    return this._data.set(window, true);
+  },
+
+  remove: function (window) {
+    this._data.delete(window);
+  },
+
+  clear: function (window) {
+    this._data = new WeakMap();
+  }
+};
+
+// The state from the previous session (after restoring pinned tabs). This
+// state is persisted and passed through to the next session during an app
+// restart to make the third party add-on warning not trash the deferred
+// session
+var LastSession = {
+  _state: null,
+
+  get canRestore() {
+    return !!this._state;
+  },
+
+  getState: function () {
+    return this._state;
+  },
+
+  setState: function (state) {
+    this._state = state;
+  },
+
+  clear: function () {
+    if (this._state) {
+      this._state = null;
+      Services.obs.notifyObservers(null, NOTIFY_LAST_SESSION_CLEARED, null);
+    }
+  }
+};