new file mode 100644
--- /dev/null
+++ b/services/sync/modules/engines/extension-storage.js
@@ -0,0 +1,71 @@
+/* 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.EXPORTED_SYMBOLS = ['ExtensionStorageEngine'];
+
+var {classes: Cc, interfaces: Ci, utils: Cu} = Components;
+
+Cu.import("resource://services-sync/constants.js");
+Cu.import("resource://services-sync/engines.js");
+Cu.import("resource://services-sync/util.js");
+Cu.import("resource://services-common/async.js");
+Cu.import("resource://gre/modules/ExtensionStorageSync.jsm");
+
+/**
+ * The Engine that manages syncing for the web extension "storage"
+ * API, and in particular ext.storage.sync.
+ *
+ * ext.storage.sync is implemented using Kinto, so it has mechanisms
+ * for syncing that we do not need to integrate in the Firefox Sync
+ * framework, so this is something of a stub.
+ */
+this.ExtensionStorageEngine = function ExtensionStorageEngine(service) {
+  SyncEngine.call(this, "Extension-Storage", service);
+}
+ExtensionStorageEngine.prototype = {
+  __proto__: SyncEngine.prototype,
+  _trackerObj: ExtensionStorageTracker,
+  // we don't need these since we implement our own sync logic
+  _storeObj: undefined,
+  _recordObj: undefined,
+
+  syncPriority: 10,
+
+  _sync: function () {
+    return ExtensionStorageSync.syncAll();
+  },
+};
+
+function ExtensionStorageTracker(name, engine) {
+  Tracker.call(this, name, engine);
+  Svc.Obs.add("weave:engine:start-tracking", this);
+  Svc.Obs.add("weave:engine:stop-tracking", this);
+}
+ExtensionStorageTracker.prototype = {
+  __proto__: Tracker.prototype,
+
+  startTracking: function () {
+    Svc.Obs.add("ext.storage.sync-changed", this);
+  },
+
+  stopTracking: function () {
+    Svc.Obs.remove("ext.storage.sync-changed", this);
+  },
+
+  observe: function (subject, topic, data) {
+    Tracker.prototype.observe.call(this, subject, topic, data);
+
+    if (this.ignoreAll) {
+      return;
+    }
+
+    if (topic !== "ext.storage.sync-changed") {
+      return;
+    }
+
+    // Single adds, removes and changes are not so important on their
+    // own, so let's just increment score a bit.
+    this.score += SCORE_INCREMENT_MEDIUM;
+  },
+};

--- a/services/sync/modules/service.js
+++ b/services/sync/modules/service.js
@@ -39,16 +39,17 @@ Cu.import("resource://services-sync/util
 const ENGINE_MODULES = {
   Addons: "addons.js",
   Bookmarks: "bookmarks.js",
   Form: "forms.js",
   History: "history.js",
   Password: "passwords.js",
   Prefs: "prefs.js",
   Tab: "tabs.js",
+  ExtensionStorage: "extension-storage.js",
 };
 
 const STORAGE_INFO_TYPES = [INFO_COLLECTIONS,
                             INFO_COLLECTION_USAGE,
                             INFO_COLLECTION_COUNTS,
                             INFO_QUOTA];
 
 function Sync11Service() {

--- a/services/sync/moz.build
+++ b/services/sync/moz.build
@@ -47,16 +47,17 @@ EXTRA_PP_JS_MODULES['services-sync'] += 
 # Definitions used by constants.js
 DEFINES['weave_version'] = '1.52.0'
 DEFINES['weave_id'] = '{340c2bbc-ce74-4362-90b5-7c26312808ef}'
 
 EXTRA_JS_MODULES['services-sync'].engines += [
     'modules/engines/addons.js',
     'modules/engines/bookmarks.js',
     'modules/engines/clients.js',
+    'modules/engines/extension-storage.js',
     'modules/engines/forms.js',
     'modules/engines/history.js',
     'modules/engines/passwords.js',
     'modules/engines/prefs.js',
     'modules/engines/tabs.js',
 ]
 
 EXTRA_JS_MODULES['services-sync'].stages += [

--- a/services/sync/services-sync.js
+++ b/services/sync/services-sync.js
@@ -26,16 +26,17 @@ pref("services.sync.errorhandler.network
 
 pref("services.sync.engine.addons", true);
 pref("services.sync.engine.bookmarks", true);
 pref("services.sync.engine.history", true);
 pref("services.sync.engine.passwords", true);
 pref("services.sync.engine.prefs", true);
 pref("services.sync.engine.tabs", true);
 pref("services.sync.engine.tabs.filteredUrls", "^(about:.*|chrome://weave/.*|wyciwyg:.*|file:.*|blob:.*)$");
+pref("services.sync.engine.extension-storage", true);
 
 pref("services.sync.jpake.serverURL", "https://setup.services.mozilla.com/");
 pref("services.sync.jpake.pollInterval", 1000);
 pref("services.sync.jpake.firstMsgMaxTries", 300); // 5 minutes
 pref("services.sync.jpake.lastMsgMaxTries", 300);  // 5 minutes
 pref("services.sync.jpake.maxTries", 10);
 
 // Allow add-ons to be synced from non-trusted sources.

new file mode 100644
--- /dev/null
+++ b/services/sync/tests/unit/test_extension_storage_tracker.js
@@ -0,0 +1,75 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+Cu.import("resource://services-sync/constants.js");
+Cu.import("resource://services-sync/engines.js");
+Cu.import("resource://services-sync/engines/extension-storage.js");
+Cu.import("resource://services-sync/service.js");
+Cu.import("resource://services-sync/util.js");
+Cu.import("resource://gre/modules/ExtensionStorageSync.jsm");
+Cu.import("resource://gre/modules/ExtensionUtils.jsm");
+
+var {
+  BaseContext,
+} = ExtensionUtils;
+
+Service.engineManager.register(ExtensionStorageEngine);
+const engine = Service.engineManager.get("extension-storage");
+do_get_profile();   // so we can use FxAccounts
+
+class Context extends BaseContext {
+  constructor(principal) {
+    super();
+    Object.defineProperty(this, "principal", {
+      value: principal,
+      configurable: true,
+    });
+    this.sandbox = Cu.Sandbox(principal, {wantXrays: false});
+    this.extension = {id: "test@web.extension"};
+  }
+
+  get cloneScope() {
+    return this.sandbox;
+  }
+}
+
+function* withContext(f) {
+  const STORAGE_SYNC_PREF = "extension.storage.sync.enabled";
+  const ssm = Services.scriptSecurityManager;
+  const PRINCIPAL1 = ssm.createCodebasePrincipalFromOrigin("http://www.example.org");
+  const context = new Context(PRINCIPAL1);
+
+  let prefs = Services.prefs;
+  try {
+    prefs.setBoolPref(STORAGE_SYNC_PREF, true);
+    yield* f(context);
+  } finally {
+    prefs.clearUserPref(STORAGE_SYNC_PREF);
+    context.unload();
+  }
+
+}
+
+add_task(function* test_changing_extension_storage_changes_score() {
+  const tracker = engine._tracker;
+  const extensionId = "my-extension-id";
+  Svc.Obs.notify("weave:engine:start-tracking");
+  yield* withContext(function*(context) {
+    yield ExtensionStorageSync.set(extensionId, {"a": "b"}, context);
+  });
+  do_check_eq(tracker.score, SCORE_INCREMENT_MEDIUM);
+
+  tracker.resetScore();
+  yield* withContext(function*(context) {
+    yield ExtensionStorageSync.remove(extensionId, "a", context);
+  });
+  do_check_eq(tracker.score, SCORE_INCREMENT_MEDIUM);
+
+  Svc.Obs.notify("weave:engine:stop-tracking");
+});
+
+function run_test() {
+  run_next_test();
+}

--- a/services/sync/tests/unit/test_load_modules.js
+++ b/services/sync/tests/unit/test_load_modules.js
@@ -8,16 +8,17 @@ const modules = [
   "constants.js",
   "engines/addons.js",
   "engines/bookmarks.js",
   "engines/clients.js",
   "engines/forms.js",
   "engines/history.js",
   "engines/passwords.js",
   "engines/prefs.js",
+  "engines/storage-sync.js",
   "engines/tabs.js",
   "engines.js",
   "identity.js",
   "jpakeclient.js",
   "keys.js",
   "main.js",
   "policies.js",
   "record.js",

--- a/services/sync/tests/unit/xpcshell.ini
+++ b/services/sync/tests/unit/xpcshell.ini
@@ -153,16 +153,17 @@ tags = addons
 [test_bookmark_smart_bookmarks.js]
 [test_bookmark_store.js]
 # Too many intermittent "ASSERTION: thread pool wasn't shutdown: '!mPool'" (bug 804479)
 skip-if = debug
 [test_bookmark_tracker.js]
 [test_bookmark_validator.js]
 [test_clients_engine.js]
 [test_clients_escape.js]
+[test_extension_storage_tracker.js]
 [test_forms_store.js]
 [test_forms_tracker.js]
 # Too many intermittent "ASSERTION: thread pool wasn't shutdown: '!mPool'" (bug 804479)
 skip-if = debug
 [test_history_engine.js]
 [test_history_store.js]
 [test_history_tracker.js]
 # Too many intermittent "ASSERTION: thread pool wasn't shutdown: '!mPool'" (bug 804479)

--- a/toolkit/components/extensions/ExtensionStorageSync.jsm
+++ b/toolkit/components/extensions/ExtensionStorageSync.jsm
@@ -18,16 +18,18 @@ const STORAGE_SYNC_ENABLED_PREF = "exten
 Cu.import("resource://gre/modules/XPCOMUtils.jsm");
 
 XPCOMUtils.defineLazyModuleGetter(this, "loadKinto",
                                   "resource://services-common/kinto-offline-client.js");
 XPCOMUtils.defineLazyModuleGetter(this, "ExtensionStorage",
                                   "resource://gre/modules/ExtensionStorage.jsm");
 XPCOMUtils.defineLazyModuleGetter(this, "AppsUtils",
                                   "resource://gre/modules/AppsUtils.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "Observers",
+                                  "resource://services-common/observers.js");
 XPCOMUtils.defineLazyPreferenceGetter(this, "prefPermitsStorageSync",
                                       STORAGE_SYNC_ENABLED_PREF, false);
 
 /* globals ExtensionStorageSync */
 
 var collectionPromises = new Map();
 
 // An "id schema" used to validate Kinto IDs and generate new ones.
@@ -248,16 +250,17 @@ this.ExtensionStorageSync = {
   },
 
   removeOnChangedListener(extensionId, listener) {
     let listeners = this.listeners.get(extensionId);
     listeners.delete(listener);
   },
 
   notifyListeners(extensionId, changes) {
+    Observers.notify("ext.storage.sync-changed");
     let listeners = this.listeners.get(extensionId);
     if (listeners) {
       for (let listener of listeners) {
         try {
           listener(changes);
         } catch(e) {
           Cu.reportError(e);
         }