diff -r 000000000000 -r 6474c204b198 dom/settings/SettingsChangeNotifier.jsm
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/dom/settings/SettingsChangeNotifier.jsm	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,123 @@
+/* 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"
+
+const DEBUG = false;
+function debug(s) {
+  if (DEBUG) dump("-*- SettingsChangeNotifier: " + s + "\n");
+}
+
+const Cu = Components.utils;
+const Cc = Components.classes;
+const Ci = Components.interfaces;
+
+this.EXPORTED_SYMBOLS = ["SettingsChangeNotifier"];
+
+Cu.import("resource://gre/modules/XPCOMUtils.jsm");
+Cu.import("resource://gre/modules/Services.jsm");
+
+const kXpcomShutdownObserverTopic      = "xpcom-shutdown";
+const kMozSettingsChangedObserverTopic = "mozsettings-changed";
+const kFromSettingsChangeNotifier      = "fromSettingsChangeNotifier";
+
+XPCOMUtils.defineLazyServiceGetter(this, "ppmm",
+                                   "@mozilla.org/parentprocessmessagemanager;1",
+                                   "nsIMessageBroadcaster");
+
+this.SettingsChangeNotifier = {
+  init: function() {
+    if (DEBUG) debug("init");
+    this.children = [];
+    this._messages = ["Settings:Changed", "Settings:RegisterForMessages", "child-process-shutdown"];
+    this._messages.forEach((function(msgName) {
+      ppmm.addMessageListener(msgName, this);
+    }).bind(this));
+
+    Services.obs.addObserver(this, kXpcomShutdownObserverTopic, false);
+    Services.obs.addObserver(this, kMozSettingsChangedObserverTopic, false);
+  },
+
+  observe: function(aSubject, aTopic, aData) {
+    if (DEBUG) debug("observe");
+    switch (aTopic) {
+      case kXpcomShutdownObserverTopic:
+        this._messages.forEach((function(msgName) {
+          ppmm.removeMessageListener(msgName, this);
+        }).bind(this));
+        Services.obs.removeObserver(this, kXpcomShutdownObserverTopic);
+        Services.obs.removeObserver(this, kMozSettingsChangedObserverTopic);
+        ppmm = null;
+        break;
+      case kMozSettingsChangedObserverTopic:
+      {
+        let setting = JSON.parse(aData);
+        // To avoid redundantly broadcasting settings-changed events that are
+        // just requested from content processes themselves, skip the observer
+        // messages that are notified from the internal SettingsChangeNotifier.
+        if (setting.message && setting.message === kFromSettingsChangeNotifier)
+          return;
+        this.broadcastMessage("Settings:Change:Return:OK",
+          { key: setting.key, value: setting.value });
+        break;
+      }
+      default:
+        if (DEBUG) debug("Wrong observer topic: " + aTopic);
+        break;
+    }
+  },
+
+  broadcastMessage: function broadcastMessage(aMsgName, aContent) {
+    if (DEBUG) debug("Broadast");
+    this.children.forEach(function(msgMgr) {
+      msgMgr.sendAsyncMessage(aMsgName, aContent);
+    });
+  },
+
+  receiveMessage: function(aMessage) {
+    if (DEBUG) debug("receiveMessage");
+    let msg = aMessage.data;
+    let mm = aMessage.target;
+    switch (aMessage.name) {
+      case "Settings:Changed":
+        if (!aMessage.target.assertPermission("settings-write")) {
+          Cu.reportError("Settings message " + msg.name +
+                         " from a content process with no 'settings-write' privileges.");
+          return null;
+        }
+        this.broadcastMessage("Settings:Change:Return:OK",
+          { key: msg.key, value: msg.value });
+        Services.obs.notifyObservers(this, kMozSettingsChangedObserverTopic,
+          JSON.stringify({
+            key: msg.key,
+            value: msg.value,
+            message: kFromSettingsChangeNotifier
+          }));
+        break;
+      case "Settings:RegisterForMessages":
+        if (!aMessage.target.assertPermission("settings-read")) {
+          Cu.reportError("Settings message " + msg.name +
+                         " from a content process with no 'settings-read' privileges.");
+          return null;
+        }
+        if (DEBUG) debug("Register!");
+        if (this.children.indexOf(mm) == -1) {
+          this.children.push(mm);
+        }
+        break;
+      case "child-process-shutdown":
+        if (DEBUG) debug("Unregister");
+        let index;
+        if ((index = this.children.indexOf(mm)) != -1) {
+          if (DEBUG) debug("Unregister index: " + index);
+          this.children.splice(index, 1);
+        }
+        break;
+      default:
+        if (DEBUG) debug("Wrong message: " + aMessage.name);
+    }
+  }
+}
+
+SettingsChangeNotifier.init();