diff -r 000000000000 -r 6474c204b198 toolkit/modules/WindowsPrefSync.jsm
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/toolkit/modules/WindowsPrefSync.jsm	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,173 @@
+/* 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 = [];
+
+#ifdef XP_WIN
+#ifdef MOZ_METRO
+
+const {classes: Cc, interfaces: Ci, results: Cr, utils: Cu, manager: Cm} =
+  Components;
+const PREF_BASE_KEY = "Software\\Mozilla\\Firefox\\Metro\\Prefs\\";
+
+Cu.import("resource://gre/modules/XPCOMUtils.jsm");
+Cu.import("resource://gre/modules/Services.jsm");
+
+this.EXPORTED_SYMBOLS = [ "WindowsPrefSync" ];
+
+/**
+ * Manages preferences that need to be pulled and pushed between Metro
+ * and desktop.
+ */
+this.WindowsPrefSync = {
+  init: function() {
+    this.pullSharedPrefs();
+    this.prefListToPush.forEach(function(prefName) {
+      this.pushSharedPref(prefName);
+      Services.prefs.addObserver(prefName, this, false);
+    }, this);
+  },
+
+  uninit: function() {
+    this.prefListToPush.forEach(function(prefName) {
+        Services.prefs.removeObserver(prefName, this);
+    }, this);
+  },
+
+  /**
+   * Returns the list of prefs that should be pushed for the current
+   * environment.
+   */
+  get prefListToPush() {
+    return !Services.metro.immersive ? this.desktopControlledPrefs :
+      this.metroControlledPrefs;
+  },
+
+  /**
+   * Returns the list of prefs that should be pulled for the current
+   * environment.
+   */
+  get prefListToPull() {
+    return Services.metro.immersive ? this.desktopControlledPrefs :
+      this.metroControlledPrefs;
+  },
+
+  /**
+   * The following preferences will be pushed to registry from Desktop
+   * Firefox and pulled in from Metro Firefox.
+   *
+   * app.update.* prefs are because Metro shares an installation directory with
+   * Firefox, and the options for these are only present in the Desktop options.
+   *
+   * browser.sessionstore.resume_session_once is mainly for the switch to Metro
+   * and switch to Desktop feature.
+   *
+   * browser.startup.page - if a desktop Firefox user wants her/his sessions
+   * to always restore, we need to honor that in metro Firefox as well.
+   */
+  desktopControlledPrefs: ["app.update.auto",
+    "app.update.enabled",
+    "app.update.service.enabled",
+    "app.update.metro.enabled",
+    "browser.sessionstore.resume_session_once",
+    "browser.startup.page"],
+
+  /**
+   * Returns the base path where registry sync prefs are stored.
+   */
+  get prefRegistryPath() {
+    let profileService = Cc["@mozilla.org/toolkit/profile-service;1"].
+      createInstance(Ci.nsIToolkitProfileService);
+    return PREF_BASE_KEY + profileService.selectedProfile.name + "\\";
+  },
+
+  /**
+   * The following preferences will be pushed to registry from Metro
+   * Firefox and pulled in from Desktop Firefox.
+   *
+   * browser.sessionstore.resume_session_once is mainly for the switch to Metro
+   * and switch to Desktop feature.
+   */
+  metroControlledPrefs: ["browser.sessionstore.resume_session_once"],
+
+  /**
+   * Observes preference changes and writes them to the registry, only
+   * the list of preferences initialized will be observed
+   */
+  observe: function (aSubject, aTopic, aPrefName) {
+    if (aTopic != "nsPref:changed")
+      return;
+
+    this.pushSharedPref(aPrefName);
+  },
+
+  /**
+   * Writes the pref to HKCU in the registry and adds a pref-observer to keep
+   * the registry in sync with changes to the value.
+   */
+  pushSharedPref : function(aPrefName) {
+    let registry = Cc["@mozilla.org/windows-registry-key;1"].
+      createInstance(Ci.nsIWindowsRegKey);
+    try {
+      var prefType = Services.prefs.getPrefType(aPrefName);
+      let prefFunc;
+      if (prefType == Ci.nsIPrefBranch.PREF_INT)
+        prefFunc = "getIntPref";
+      else if (prefType == Ci.nsIPrefBranch.PREF_BOOL)
+        prefFunc = "getBoolPref";
+      else if (prefType == Ci.nsIPrefBranch.PREF_STRING)
+        prefFunc = "getCharPref";
+      else
+        throw "Unsupported pref type";
+
+      let prefValue = Services.prefs[prefFunc](aPrefName);
+      registry.create(Ci.nsIWindowsRegKey.ROOT_KEY_CURRENT_USER,
+        this.prefRegistryPath + prefType, Ci.nsIWindowsRegKey.ACCESS_WRITE);
+      // Always write as string, but the registry subfolder will determine
+      // how Metro interprets that string value.
+      registry.writeStringValue(aPrefName, prefValue);
+    } catch (ex) {
+      Cu.reportError("Couldn't push pref " + aPrefName + ": " + ex);
+    } finally {
+      registry.close();
+    }
+  },
+
+  /**
+   * Pulls in all shared prefs from the registry
+   */
+  pullSharedPrefs: function() {
+    function pullSharedPrefType(prefType, prefFunc) {
+      try {
+        registry.create(Ci.nsIWindowsRegKey.ROOT_KEY_CURRENT_USER,
+          self.prefRegistryPath + prefType,
+          Ci.nsIWindowsRegKey.ACCESS_ALL);
+        for (let i = 0; i < registry.valueCount; i++) {
+          let prefName = registry.getValueName(i);
+          let prefValue = registry.readStringValue(prefName);
+          if (prefType == Ci.nsIPrefBranch.PREF_BOOL) {
+            prefValue = prefValue == "true";
+          }
+          if (self.prefListToPull.indexOf(prefName) != -1) {
+            Services.prefs[prefFunc](prefName, prefValue);
+          }
+        }
+      } catch (ex) {
+        dump("Could not pull for prefType " + prefType + ": " + ex + "\n");
+      } finally {
+        registry.close();
+      }
+    }
+    let self = this;
+    let registry = Cc["@mozilla.org/windows-registry-key;1"].
+      createInstance(Ci.nsIWindowsRegKey);
+    pullSharedPrefType(Ci.nsIPrefBranch.PREF_INT, "setIntPref");
+    pullSharedPrefType(Ci.nsIPrefBranch.PREF_BOOL, "setBoolPref");
+    pullSharedPrefType(Ci.nsIPrefBranch.PREF_STRING, "setCharPref");
+  }
+};
+#endif
+#endif