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diff --git a/browser/modules/NetworkPrioritizer.jsm b/browser/modules/NetworkPrioritizer.jsm
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+/* 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/. */
+
+/*
+ * This module adjusts network priority for tabs in a way that gives 'important'
+ * tabs a higher priority. There are 3 levels of priority. Each is listed below
+ * with the priority adjustment used.
+ *
+ * Highest (-10): Selected tab in the focused window.
+ * Medium (0):    Background tabs in the focused window.
+ *                Selected tab in background windows.
+ * Lowest (+10):  Background tabs in background windows.
+ */
+
+this.EXPORTED_SYMBOLS = ["trackBrowserWindow"];
+
+const Ci = Components.interfaces;
+
+Components.utils.import("resource://gre/modules/XPCOMUtils.jsm");
+
+
+// Lazy getters
+XPCOMUtils.defineLazyServiceGetter(this, "_focusManager",
+                                   "@mozilla.org/focus-manager;1",
+                                   "nsIFocusManager");
+
+
+// Constants
+const TAB_EVENTS = ["TabBrowserInserted", "TabSelect", "TabRemotenessChange"];
+const WINDOW_EVENTS = ["activate", "unload"];
+// lower value means higher priority
+const PRIORITY_DELTA = Ci.nsISupportsPriority.PRIORITY_NORMAL - Ci.nsISupportsPriority.PRIORITY_LOW;
+
+
+// Variables
+var _lastFocusedWindow = null;
+var _windows = [];
+// this is used for restoring the priority after TabRemotenessChange
+var _priorityBackup = new WeakMap();
+
+
+// Exported symbol
+this.trackBrowserWindow = function trackBrowserWindow(aWindow) {
+  WindowHelper.addWindow(aWindow);
+}
+
+
+// Global methods
+function _handleEvent(aEvent) {
+  switch (aEvent.type) {
+    case "TabBrowserInserted":
+      BrowserHelper.onOpen(aEvent.target.linkedBrowser);
+      break;
+    case "TabSelect":
+      BrowserHelper.onSelect(aEvent.target.linkedBrowser);
+      break;
+    case "activate":
+      WindowHelper.onActivate(aEvent.target);
+      break;
+    case "TabRemotenessChange":
+      BrowserHelper.onRemotenessChange(aEvent.target.linkedBrowser);
+      break;
+    case "unload":
+      WindowHelper.removeWindow(aEvent.currentTarget);
+      break;
+  }
+}
+
+
+// Methods that impact a browser. Put into single object for organization.
+var BrowserHelper = {
+  onOpen: function NP_BH_onOpen(aBrowser) {
+    _priorityBackup.set(aBrowser.permanentKey, Ci.nsISupportsPriority.PRIORITY_NORMAL);
+
+    // If the tab is in the focused window, leave priority as it is
+    if (aBrowser.ownerGlobal != _lastFocusedWindow)
+      this.decreasePriority(aBrowser);
+  },
+
+  onSelect: function NP_BH_onSelect(aBrowser) {
+    let windowEntry = WindowHelper.getEntry(aBrowser.ownerGlobal);
+    if (windowEntry.lastSelectedBrowser)
+      this.decreasePriority(windowEntry.lastSelectedBrowser);
+    this.increasePriority(aBrowser);
+
+    windowEntry.lastSelectedBrowser = aBrowser;
+  },
+
+  onRemotenessChange: function (aBrowser) {
+    aBrowser.setPriority(_priorityBackup.get(aBrowser.permanentKey));
+  },
+
+  increasePriority: function NP_BH_increasePriority(aBrowser) {
+    aBrowser.adjustPriority(PRIORITY_DELTA);
+    _priorityBackup.set(aBrowser.permanentKey,
+                        _priorityBackup.get(aBrowser.permanentKey) + PRIORITY_DELTA);
+  },
+
+  decreasePriority: function NP_BH_decreasePriority(aBrowser) {
+    aBrowser.adjustPriority(PRIORITY_DELTA * -1);
+    _priorityBackup.set(aBrowser.permanentKey,
+                        _priorityBackup.get(aBrowser.permanentKey) - PRIORITY_DELTA);
+  }
+};
+
+
+// Methods that impact a window. Put into single object for organization.
+var WindowHelper = {
+  addWindow: function NP_WH_addWindow(aWindow) {
+    // Build internal data object
+    _windows.push({ window: aWindow, lastSelectedBrowser: null });
+
+    // Add event listeners
+    TAB_EVENTS.forEach(function(event) {
+      aWindow.gBrowser.tabContainer.addEventListener(event, _handleEvent, false);
+    });
+    WINDOW_EVENTS.forEach(function(event) {
+      aWindow.addEventListener(event, _handleEvent, false);
+    });
+
+    // This gets called AFTER activate event, so if this is the focused window
+    // we want to activate it. Otherwise, deprioritize it.
+    if (aWindow == _focusManager.activeWindow)
+      this.handleFocusedWindow(aWindow);
+    else
+      this.decreasePriority(aWindow);
+
+    // Select the selected tab
+    BrowserHelper.onSelect(aWindow.gBrowser.selectedBrowser);
+  },
+
+  removeWindow: function NP_WH_removeWindow(aWindow) {
+    if (aWindow == _lastFocusedWindow)
+      _lastFocusedWindow = null;
+
+    // Delete this window from our tracking
+    _windows.splice(this.getEntryIndex(aWindow), 1);
+
+    // Remove the event listeners
+    TAB_EVENTS.forEach(function(event) {
+      aWindow.gBrowser.tabContainer.removeEventListener(event, _handleEvent, false);
+    });
+    WINDOW_EVENTS.forEach(function(event) {
+      aWindow.removeEventListener(event, _handleEvent, false);
+    });
+  },
+
+  onActivate: function NP_WH_onActivate(aWindow, aHasFocus) {
+    // If this window was the last focused window, we don't need to do anything
+    if (aWindow == _lastFocusedWindow)
+      return;
+
+    // handleFocusedWindow will deprioritize the current window
+    this.handleFocusedWindow(aWindow);
+
+    // Lastly we should increase priority for this window
+    this.increasePriority(aWindow);
+  },
+
+  handleFocusedWindow: function NP_WH_handleFocusedWindow(aWindow) {
+    // If we have a last focused window, we need to deprioritize it first
+    if (_lastFocusedWindow)
+      this.decreasePriority(_lastFocusedWindow);
+
+    // aWindow is now focused
+    _lastFocusedWindow = aWindow;
+  },
+
+  // Auxiliary methods
+  increasePriority: function NP_WH_increasePriority(aWindow) {
+    aWindow.gBrowser.browsers.forEach(function(aBrowser) {
+      BrowserHelper.increasePriority(aBrowser);
+    });
+  },
+
+  decreasePriority: function NP_WH_decreasePriority(aWindow) {
+    aWindow.gBrowser.browsers.forEach(function(aBrowser) {
+      BrowserHelper.decreasePriority(aBrowser);
+    });
+  },
+
+  getEntry: function NP_WH_getEntry(aWindow) {
+    return _windows[this.getEntryIndex(aWindow)];
+  },
+
+  getEntryIndex: function NP_WH_getEntryAtIndex(aWindow) {
+    // Assumes that every object has a unique window & it's in the array
+    for (let i = 0; i < _windows.length; i++)
+      if (_windows[i].window == aWindow)
+        return i;
+  }
+};
+