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diff --git a/browser/components/sessionstore/nsSessionStartup.js b/browser/components/sessionstore/nsSessionStartup.js
new file mode 100644
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+++ b/browser/components/sessionstore/nsSessionStartup.js
@@ -0,0 +1,353 @@
+/* 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";
+
+/**
+ * Session Storage and Restoration
+ *
+ * Overview
+ * This service reads user's session file at startup, and makes a determination
+ * as to whether the session should be restored. It will restore the session
+ * under the circumstances described below.  If the auto-start Private Browsing
+ * mode is active, however, the session is never restored.
+ *
+ * Crash Detection
+ * The CrashMonitor is used to check if the final session state was successfully
+ * written at shutdown of the last session. If we did not reach
+ * 'sessionstore-final-state-write-complete', then it's assumed that the browser
+ * has previously crashed and we should restore the session.
+ *
+ * Forced Restarts
+ * In the event that a restart is required due to application update or extension
+ * installation, set the browser.sessionstore.resume_session_once pref to true,
+ * and the session will be restored the next time the browser starts.
+ *
+ * Always Resume
+ * This service will always resume the session if the integer pref
+ * browser.startup.page is set to 3.
+ */
+
+/* :::::::: Constants and Helpers ::::::::::::::: */
+
+const Cc = Components.classes;
+const Ci = Components.interfaces;
+const Cr = Components.results;
+const Cu = Components.utils;
+Cu.import("resource://gre/modules/XPCOMUtils.jsm");
+Cu.import("resource://gre/modules/Services.jsm");
+Cu.import("resource://gre/modules/TelemetryStopwatch.jsm");
+Cu.import("resource://gre/modules/PrivateBrowsingUtils.jsm");
+Cu.import("resource://gre/modules/Promise.jsm");
+
+XPCOMUtils.defineLazyModuleGetter(this, "console",
+  "resource://gre/modules/Console.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "SessionFile",
+  "resource:///modules/sessionstore/SessionFile.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "StartupPerformance",
+  "resource:///modules/sessionstore/StartupPerformance.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "CrashMonitor",
+  "resource://gre/modules/CrashMonitor.jsm");
+
+const STATE_RUNNING_STR = "running";
+
+// 'browser.startup.page' preference value to resume the previous session.
+const BROWSER_STARTUP_RESUME_SESSION = 3;
+
+function debug(aMsg) {
+  aMsg = ("SessionStartup: " + aMsg).replace(/\S{80}/g, "$&\n");
+  Services.console.logStringMessage(aMsg);
+}
+function warning(aMsg, aException) {
+  let consoleMsg = Cc["@mozilla.org/scripterror;1"].createInstance(Ci.nsIScriptError);
+consoleMsg.init(aMsg, aException.fileName, null, aException.lineNumber, 0, Ci.nsIScriptError.warningFlag, "component javascript");
+  Services.console.logMessage(consoleMsg);
+}
+
+var gOnceInitializedDeferred = (function () {
+  let deferred = {};
+
+  deferred.promise = new Promise((resolve, reject) => {
+    deferred.resolve = resolve;
+    deferred.reject = reject;
+  });
+
+  return deferred;
+})();
+
+/* :::::::: The Service ::::::::::::::: */
+
+function SessionStartup() {
+}
+
+SessionStartup.prototype = {
+
+  // the state to restore at startup
+  _initialState: null,
+  _sessionType: Ci.nsISessionStartup.NO_SESSION,
+  _initialized: false,
+
+  // Stores whether the previous session crashed.
+  _previousSessionCrashed: null,
+
+/* ........ Global Event Handlers .............. */
+
+  /**
+   * Initialize the component
+   */
+  init: function sss_init() {
+    Services.obs.notifyObservers(null, "sessionstore-init-started", null);
+    StartupPerformance.init();
+
+    // do not need to initialize anything in auto-started private browsing sessions
+    if (PrivateBrowsingUtils.permanentPrivateBrowsing) {
+      this._initialized = true;
+      gOnceInitializedDeferred.resolve();
+      return;
+    }
+
+    SessionFile.read().then(
+      this._onSessionFileRead.bind(this),
+      console.error
+    );
+  },
+
+  // Wrap a string as a nsISupports
+  _createSupportsString: function ssfi_createSupportsString(aData) {
+    let string = Cc["@mozilla.org/supports-string;1"]
+                   .createInstance(Ci.nsISupportsString);
+    string.data = aData;
+    return string;
+  },
+
+  /**
+   * Complete initialization once the Session File has been read
+   *
+   * @param source The Session State string read from disk.
+   * @param parsed The object obtained by parsing |source| as JSON.
+   */
+  _onSessionFileRead: function ({source, parsed, noFilesFound}) {
+    this._initialized = true;
+
+    // Let observers modify the state before it is used
+    let supportsStateString = this._createSupportsString(source);
+    Services.obs.notifyObservers(supportsStateString, "sessionstore-state-read", "");
+    let stateString = supportsStateString.data;
+
+    if (stateString != source) {
+      // The session has been modified by an add-on, reparse.
+      try {
+        this._initialState = JSON.parse(stateString);
+      } catch (ex) {
+        // That's not very good, an add-on has rewritten the initial
+        // state to something that won't parse.
+        warning("Observer rewrote the state to something that won't parse", ex);
+      }
+    } else {
+      // No need to reparse
+      this._initialState = parsed;
+    }
+
+    if (this._initialState == null) {
+      // No valid session found.
+      this._sessionType = Ci.nsISessionStartup.NO_SESSION;
+      Services.obs.notifyObservers(null, "sessionstore-state-finalized", "");
+      gOnceInitializedDeferred.resolve();
+      return;
+    }
+
+    let shouldResumeSessionOnce = Services.prefs.getBoolPref("browser.sessionstore.resume_session_once");
+    let shouldResumeSession = shouldResumeSessionOnce ||
+          Services.prefs.getIntPref("browser.startup.page") == BROWSER_STARTUP_RESUME_SESSION;
+
+    // If this is a normal restore then throw away any previous session
+    if (!shouldResumeSessionOnce && this._initialState) {
+      delete this._initialState.lastSessionState;
+    }
+
+    let resumeFromCrash = Services.prefs.getBoolPref("browser.sessionstore.resume_from_crash");
+
+    CrashMonitor.previousCheckpoints.then(checkpoints => {
+      if (checkpoints) {
+        // If the previous session finished writing the final state, we'll
+        // assume there was no crash.
+        this._previousSessionCrashed = !checkpoints["sessionstore-final-state-write-complete"];
+
+      } else {
+        // If the Crash Monitor could not load a checkpoints file it will
+        // provide null. This could occur on the first run after updating to
+        // a version including the Crash Monitor, or if the checkpoints file
+        // was removed, or on first startup with this profile, or after Firefox Reset.
+
+        if (noFilesFound) {
+          // There was no checkpoints file and no sessionstore.js or its backups
+          // so we will assume that this was a fresh profile.
+          this._previousSessionCrashed = false;
+
+        } else {
+          // If this is the first run after an update, sessionstore.js should
+          // still contain the session.state flag to indicate if the session
+          // crashed. If it is not present, we will assume this was not the first
+          // run after update and the checkpoints file was somehow corrupted or
+          // removed by a crash.
+          //
+          // If the session.state flag is present, we will fallback to using it
+          // for crash detection - If the last write of sessionstore.js had it
+          // set to "running", we crashed.
+          let stateFlagPresent = (this._initialState.session &&
+                                  this._initialState.session.state);
+
+
+          this._previousSessionCrashed = !stateFlagPresent ||
+            (this._initialState.session.state == STATE_RUNNING_STR);
+        }
+      }
+
+      // Report shutdown success via telemetry. Shortcoming here are
+      // being-killed-by-OS-shutdown-logic, shutdown freezing after
+      // session restore was written, etc.
+      Services.telemetry.getHistogramById("SHUTDOWN_OK").add(!this._previousSessionCrashed);
+
+      // set the startup type
+      if (this._previousSessionCrashed && resumeFromCrash)
+        this._sessionType = Ci.nsISessionStartup.RECOVER_SESSION;
+      else if (!this._previousSessionCrashed && shouldResumeSession)
+        this._sessionType = Ci.nsISessionStartup.RESUME_SESSION;
+      else if (this._initialState)
+        this._sessionType = Ci.nsISessionStartup.DEFER_SESSION;
+      else
+        this._initialState = null; // reset the state
+
+      Services.obs.addObserver(this, "sessionstore-windows-restored", true);
+
+      if (this._sessionType != Ci.nsISessionStartup.NO_SESSION)
+        Services.obs.addObserver(this, "browser:purge-session-history", true);
+
+      // We're ready. Notify everyone else.
+      Services.obs.notifyObservers(null, "sessionstore-state-finalized", "");
+      gOnceInitializedDeferred.resolve();
+    });
+  },
+
+  /**
+   * Handle notifications
+   */
+  observe: function sss_observe(aSubject, aTopic, aData) {
+    switch (aTopic) {
+    case "app-startup":
+      Services.obs.addObserver(this, "final-ui-startup", true);
+      Services.obs.addObserver(this, "quit-application", true);
+      break;
+    case "final-ui-startup":
+      Services.obs.removeObserver(this, "final-ui-startup");
+      Services.obs.removeObserver(this, "quit-application");
+      this.init();
+      break;
+    case "quit-application":
+      // no reason for initializing at this point (cf. bug 409115)
+      Services.obs.removeObserver(this, "final-ui-startup");
+      Services.obs.removeObserver(this, "quit-application");
+      if (this._sessionType != Ci.nsISessionStartup.NO_SESSION)
+        Services.obs.removeObserver(this, "browser:purge-session-history");
+      break;
+    case "sessionstore-windows-restored":
+      Services.obs.removeObserver(this, "sessionstore-windows-restored");
+      // free _initialState after nsSessionStore is done with it
+      this._initialState = null;
+      break;
+    case "browser:purge-session-history":
+      Services.obs.removeObserver(this, "browser:purge-session-history");
+      // reset all state on sanitization
+      this._sessionType = Ci.nsISessionStartup.NO_SESSION;
+      break;
+    }
+  },
+
+/* ........ Public API ................*/
+
+  get onceInitialized() {
+    return gOnceInitializedDeferred.promise;
+  },
+
+  /**
+   * Get the session state as a jsval
+   */
+  get state() {
+    return this._initialState;
+  },
+
+  /**
+   * Determines whether there is a pending session restore. Should only be
+   * called after initialization has completed.
+   * @returns bool
+   */
+  doRestore: function sss_doRestore() {
+    return this._willRestore();
+  },
+
+  /**
+   * Determines whether automatic session restoration is enabled for this
+   * launch of the browser. This does not include crash restoration. In
+   * particular, if session restore is configured to restore only in case of
+   * crash, this method returns false.
+   * @returns bool
+   */
+  isAutomaticRestoreEnabled: function () {
+    return Services.prefs.getBoolPref("browser.sessionstore.resume_session_once") ||
+           Services.prefs.getIntPref("browser.startup.page") == BROWSER_STARTUP_RESUME_SESSION;
+  },
+
+  /**
+   * Determines whether there is a pending session restore.
+   * @returns bool
+   */
+  _willRestore: function () {
+    return this._sessionType == Ci.nsISessionStartup.RECOVER_SESSION ||
+           this._sessionType == Ci.nsISessionStartup.RESUME_SESSION;
+  },
+
+  /**
+   * Returns whether we will restore a session that ends up replacing the
+   * homepage. The browser uses this to not start loading the homepage if
+   * we're going to stop its load anyway shortly after.
+   *
+   * This is meant to be an optimization for the average case that loading the
+   * session file finishes before we may want to start loading the default
+   * homepage. Should this be called before the session file has been read it
+   * will just return false.
+   *
+   * @returns bool
+   */
+  get willOverrideHomepage() {
+    if (this._initialState && this._willRestore()) {
+      let windows = this._initialState.windows || null;
+      // If there are valid windows with not only pinned tabs, signal that we
+      // will override the default homepage by restoring a session.
+      return windows && windows.some(w => w.tabs.some(t => !t.pinned));
+    }
+    return false;
+  },
+
+  /**
+   * Get the type of pending session store, if any.
+   */
+  get sessionType() {
+    return this._sessionType;
+  },
+
+  /**
+   * Get whether the previous session crashed.
+   */
+  get previousSessionCrashed() {
+    return this._previousSessionCrashed;
+  },
+
+  /* ........ QueryInterface .............. */
+  QueryInterface : XPCOMUtils.generateQI([Ci.nsIObserver,
+                                          Ci.nsISupportsWeakReference,
+                                          Ci.nsISessionStartup]),
+  classID:          Components.ID("{ec7a6c20-e081-11da-8ad9-0800200c9a66}")
+};
+
+this.NSGetFactory = XPCOMUtils.generateNSGetFactory([SessionStartup]);