diff --git a/dom/base/Navigator.cpp b/dom/base/Navigator.cpp
index 8ece24377d6..e4fb44c49b9 100644
--- a/dom/base/Navigator.cpp
+++ b/dom/base/Navigator.cpp
@@ -1847,6 +1847,33 @@ Navigator::MozHasPendingMessage(const nsAString& aType, ErrorResult& aRv)
   return result;
 }
 
+void
+Navigator::MozSetMessageHandlerPromise(Promise& aPromise,
+                                       ErrorResult& aRv)
+{
+  // The WebIDL binding is responsible for the pref check here.
+  aRv = EnsureMessagesManager();
+  if (NS_WARN_IF(aRv.Failed())) {
+    return;
+  }
+
+  bool result = false;
+  aRv = mMessagesManager->MozIsHandlingMessage(&result);
+  if (NS_WARN_IF(aRv.Failed())) {
+    return;
+  }
+
+  if (!result) {
+    aRv.Throw(NS_ERROR_DOM_INVALID_ACCESS_ERR);
+    return;
+  }
+
+  aRv = mMessagesManager->MozSetMessageHandlerPromise(&aPromise);
+  if (NS_WARN_IF(aRv.Failed())) {
+    return;
+  }
+}
+
 void
 Navigator::MozSetMessageHandler(const nsAString& aType,
                                 systemMessageCallback* aCallback,
diff --git a/dom/base/Navigator.h b/dom/base/Navigator.h
index 3852e49c901..7d7daff15ad 100644
--- a/dom/base/Navigator.h
+++ b/dom/base/Navigator.h
@@ -235,6 +235,8 @@ public:
                             systemMessageCallback* aCallback,
                             ErrorResult& aRv);
   bool MozHasPendingMessage(const nsAString& aType, ErrorResult& aRv);
+  void MozSetMessageHandlerPromise(Promise& aPromise, ErrorResult& aRv);
+
 #ifdef MOZ_B2G
   already_AddRefed<Promise> GetMobileIdAssertion(const MobileIdOptions& options,
                                                  ErrorResult& aRv);
diff --git a/dom/messages/SystemMessageManager.js b/dom/messages/SystemMessageManager.js
index 538b9e7fa49..b9740a52d14 100644
--- a/dom/messages/SystemMessageManager.js
+++ b/dom/messages/SystemMessageManager.js
@@ -42,6 +42,10 @@ function SystemMessageManager() {
   // Flag to specify if this process has already registered the manifest URL.
   this._registerManifestURLReady = false;
 
+  // Used to know if the promise has to be accepted or not.
+  this._isHandling = false;
+  this._promise = null;
+
   // Flag to determine this process is a parent or child process.
   let appInfo = Cc["@mozilla.org/xre/app-info;1"];
   this._isParentProcess =
@@ -71,6 +75,7 @@ SystemMessageManager.prototype = {
     }
 
     aDispatcher.isHandling = true;
+    this._isHandling = true;
 
     // We get a json blob, but in some cases we want another kind of object
     // to be dispatched. To do so, we check if we have a valid contract ID of
@@ -94,15 +99,29 @@ SystemMessageManager.prototype = {
       .handleMessage(wrapped ? aMessage
                              : Cu.cloneInto(aMessage, this._window));
 
-    // We need to notify the parent one of the system messages has been handled,
-    // so the parent can release the CPU wake lock it took on our behalf.
-    cpmm.sendAsyncMessage("SystemMessageManager:HandleMessageDone",
-                          { type: aType,
-                            manifestURL: this._manifestURL,
-                            pageURL: this._pageURL,
-                            msgID: aMessageID });
-
     aDispatcher.isHandling = false;
+    this._isHandling = false;
+
+    let self = this;
+    function sendResponse() {
+      // We need to notify the parent one of the system messages has been
+      // handled, so the parent can release the CPU wake lock it took on our
+      // behalf.
+      cpmm.sendAsyncMessage("SystemMessageManager:HandleMessageDone",
+                            { type: aType,
+                              manifestURL: self._manifestURL,
+                              pageURL: self._pageURL,
+                              msgID: aMessageID });
+    }
+
+    if (!this._promise) {
+      debug("No promise set, sending the response immediately");
+      sendResponse();
+    } else {
+      debug("Using the promise to postpone the response.");
+      this._promise.then(sendResponse, sendResponse);
+      this._promise = null;
+    }
 
     if (aDispatcher.messages.length > 0) {
       let msg = aDispatcher.messages.shift();
@@ -171,9 +190,25 @@ SystemMessageManager.prototype = {
                                   manifestURL: this._manifestURL })[0];
   },
 
+  mozIsHandlingMessage: function() {
+    debug("is handling message: " + this._isHandling);
+    return this._isHandling;
+  },
+
+  mozSetMessageHandlerPromise: function(aPromise) {
+    debug("setting a promise");
+
+    if (!this._isHandling) {
+      throw "Not in a handleMessage method";
+    }
+
+    this._promise = aPromise;
+  },
+
   uninit: function()  {
     this._dispatchers = null;
     this._pendings = null;
+    this._promise = null;
 
     if (this._isParentProcess) {
       Services.obs.removeObserver(this, kSystemMessageInternalReady);
diff --git a/dom/messages/interfaces/nsIDOMNavigatorSystemMessages.idl b/dom/messages/interfaces/nsIDOMNavigatorSystemMessages.idl
index 855cfb0c8c7..4fc156c0e9d 100644
--- a/dom/messages/interfaces/nsIDOMNavigatorSystemMessages.idl
+++ b/dom/messages/interfaces/nsIDOMNavigatorSystemMessages.idl
@@ -10,10 +10,15 @@ interface nsIDOMSystemMessageCallback : nsISupports
     void handleMessage(in jsval message);
 };
 
-[scriptable, uuid(091e90dd-0e8b-463d-8cdc-9225d3a6ff90)]
+[scriptable, uuid(d04d3c11-26aa-46eb-a981-353af101f9cf)]
 interface nsIDOMNavigatorSystemMessages : nsISupports
 {
     void mozSetMessageHandler(in DOMString type, in nsIDOMSystemMessageCallback callback);
 
     boolean mozHasPendingMessage(in DOMString type); 
+
+    // the parameter is a promise object.
+    void mozSetMessageHandlerPromise(in nsISupports promise);
+
+    bool mozIsHandlingMessage();
 };
diff --git a/dom/requestsync/RequestSyncService.jsm b/dom/requestsync/RequestSyncService.jsm
index 03df16c7140..dbd8d3422d1 100644
--- a/dom/requestsync/RequestSyncService.jsm
+++ b/dom/requestsync/RequestSyncService.jsm
@@ -587,10 +587,16 @@ this.RequestSyncService = {
       timer = null;
     }
 
+    let timeout = RSYNC_OPERATION_TIMEOUT;
+    try {
+      let tmp = Services.prefs.getIntPref("dom.requestSync.maxTaskTimeout");
+      timeout = tmp;
+    } catch(e) {}
+
     timer.initWithCallback(function() {
       debug("Task is taking too much, let's ignore the promise.");
       taskCompleted();
-    }, RSYNC_OPERATION_TIMEOUT, Ci.nsITimer.TYPE_ONE_SHOT);
+    }, timeout, Ci.nsITimer.TYPE_ONE_SHOT);
 
     // Sending the message.
     let promise =
diff --git a/dom/requestsync/tests/mochitest.ini b/dom/requestsync/tests/mochitest.ini
index b1071e0e76c..a5fee3b1064 100644
--- a/dom/requestsync/tests/mochitest.ini
+++ b/dom/requestsync/tests/mochitest.ini
@@ -14,3 +14,6 @@ support-files =
 run-if = buildapp != 'b2g'
 [test_wakeUp.html]
 run-if = buildapp == 'b2g' && toolkit == 'gonk'
+[test_promise.html]
+[test_promise_app.html]
+run-if = buildapp == 'b2g' && toolkit == 'gonk'
diff --git a/dom/requestsync/tests/test_basic_app.html b/dom/requestsync/tests/test_basic_app.html
index 21437ae3eb2..bb5baeb4f39 100644
--- a/dom/requestsync/tests/test_basic_app.html
+++ b/dom/requestsync/tests/test_basic_app.html
@@ -72,7 +72,7 @@
       SpecialPowers.pushPermissions(
         [{ "type": "browser", "allow": 1, "context": document },
          { "type": "embed-apps", "allow": 1, "context": document },
-         {"type": "requestsync-manager", "allow": 1, "context": document },
+         { "type": "requestsync-manager", "allow": 1, "context": document },
          { "type": "webapps-manage", "allow": 1, "context": document }], runTests);
     },
 
diff --git a/dom/requestsync/tests/test_promise.html b/dom/requestsync/tests/test_promise.html
new file mode 100644
index 00000000000..8277eadaa6d
--- /dev/null
+++ b/dom/requestsync/tests/test_promise.html
@@ -0,0 +1,56 @@
+<!DOCTYPE HTML>
+<html>
+<head>
+  <meta charset="utf-8">
+  <title>Test for requestSync - promise</title>
+  <script type="application/javascript" src="/tests/SimpleTest/SimpleTest.js"></script>
+  <script type="application/javascript" src="common_basic.js"></script>
+  <link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css"/>
+</head>
+<body>
+<div id="container"></div>
+  <script type="application/javascript;version=1.7">
+
+SimpleTest.waitForExplicitFinish();
+SpecialPowers.pushPrefEnv({"set": [["dom.sysmsg.enabled", true]]}, function() {
+
+  ok("mozSetMessageHandlerPromise" in navigator, "mozSetMessageHandlerPromise exists");
+
+  var status = false;
+  try {
+    navigator.mozSetMessageHandlerPromise();
+  } catch(e) {
+    status = true;
+  }
+  ok(status, "mozSetMessageHandlerPromise wants a promise 1");
+
+  status = false;
+  try {
+    navigator.mozSetMessageHandlerPromise(42);
+  } catch(e) {
+    status = true;
+  }
+  ok(status, "mozSetMessageHandlerPromise wants a promise 2");
+
+  status = false;
+  try {
+    navigator.mozSetMessageHandlerPromise("hello world");
+  } catch(e) {
+    status = true;
+  }
+  ok(status, "mozSetMessageHandlerPromise wants a promise 3");
+
+  status = false;
+  try {
+    navigator.mozSetMessageHandlerPromise(new Promise(function(a, b) {}));
+  } catch(e) {
+    info(e);
+    status = true;
+  }
+  ok(status, "mozSetMessageHandlerPromise cannot be called outside a messageHandler");
+  SimpleTest.finish();
+});
+
+  </script>
+</body>
+</html>
diff --git a/dom/requestsync/tests/test_promise_app.html b/dom/requestsync/tests/test_promise_app.html
new file mode 100644
index 00000000000..9039c8e0fba
--- /dev/null
+++ b/dom/requestsync/tests/test_promise_app.html
@@ -0,0 +1,138 @@
+<!DOCTYPE HTML>
+<html>
+<head>
+  <meta charset="utf-8">
+  <title>Test for requestSync - promise</title>
+  <script type="application/javascript" src="/tests/SimpleTest/SimpleTest.js"></script>
+  <script type="application/javascript" src="common_basic.js"></script>
+  <link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css"/>
+</head>
+<body>
+<div id="container"></div>
+  <script type="application/javascript;version=1.7">
+
+  var foobarCounter = 0;
+  var pendingCounter = 0;
+  function setMessageHandler() {
+    navigator.mozSetMessageHandler('request-sync', function(e) {
+
+      if (e.task == 'foobar') {
+        ok(true, "foobar message received:" + foobarCounter);
+
+        if (++foobarCounter == 1) {
+          // The first time we wait 2 seconds.
+          info("Setting a promise object.");
+          navigator.mozSetMessageHandlerPromise(new Promise(function(a, b) {
+            setTimeout(a, 2000);
+          }));
+        } else {
+          // The second time we don't reply at all.
+          navigator.mozSetMessageHandlerPromise(new Promise(function(a, b) {}));
+        }
+      }
+
+      else if (e.task  == 'pending') {
+        ok(true, "pending message received: " + pendingCounter);
+        if (++pendingCounter == 2) {
+          runTests();
+        }
+      }
+
+      else {
+      ok(false, "Unknown message");
+      }
+    });
+
+    runTests();
+  }
+
+  function test_register_foobar() {
+    navigator.sync.register('foobar', { minInterval: 5,
+                                        oneShot: false,
+                                        data: 42,
+                                        wifiOnly: false,
+                                        wakeUpPage: location.href }).then(
+    function() {
+      ok(true, "navigator.sync.register() foobar done");
+      runTests();
+    }, genericError);
+  }
+
+  function test_register_pending() {
+    navigator.sync.register('pending', { minInterval: 6,
+                                         oneShot: false,
+                                         data: 'hello world!',
+                                         wifiOnly: false,
+                                         wakeUpPage: location.href }).then(
+    function() {
+      ok(true, "navigator.sync.register() pending done");
+      runTests();
+    }, genericError);
+  }
+
+  function test_unregister_foobar() {
+    navigator.sync.unregister('foobar').then(
+    function() {
+      ok(true, "navigator.sync.unregister() foobar done");
+      runTests();
+    }, genericError);
+  }
+
+  function test_unregister_pending() {
+    navigator.sync.unregister('pending').then(
+    function() {
+      ok(true, "navigator.sync.unregister() pending done");
+      runTests();
+    }, genericError);
+  }
+
+  function test_wait() {
+    // nothing to do here.
+  }
+
+  var tests = [
+    function() {
+      SpecialPowers.pushPrefEnv({"set": [["dom.requestSync.enabled", true],
+                                         ["dom.requestSync.minInterval", 1],
+                                         ["dom.requestSync.maxTaskTimeout", 10000 /* 10 seconds */],
+                                         ["dom.ignore_webidl_scope_checks", true]]}, runTests);
+    },
+
+    function() {
+      SpecialPowers.pushPermissions(
+        [{ "type": "requestsync-manager", "allow": 1, "context": document } ], runTests);
+    },
+
+    function() {
+      if (SpecialPowers.isMainProcess()) {
+        SpecialPowers.Cu.import("resource://gre/modules/RequestSyncService.jsm");
+      }
+      runTests();
+    },
+
+    setMessageHandler,
+
+    test_register_foobar,
+    test_register_pending,
+
+    test_wait,
+
+    test_unregister_foobar,
+    test_unregister_pending,
+  ];
+
+  function runTests() {
+    if (!tests.length) {
+      SimpleTest.finish();
+      return;
+    }
+
+    var test = tests.shift();
+    test();
+  }
+
+  SimpleTest.waitForExplicitFinish();
+  runTests();
+  </script>
+</body>
+</html>
diff --git a/dom/requestsync/tests/test_wakeUp.html b/dom/requestsync/tests/test_wakeUp.html
index f19c4f3a150..14d7ec6040c 100644
--- a/dom/requestsync/tests/test_wakeUp.html
+++ b/dom/requestsync/tests/test_wakeUp.html
@@ -84,6 +84,22 @@
     }, genericError);
   }
 
+  function test_unregister_oneShot() {
+    navigator.sync.unregister('oneShot').then(
+    function() {
+      ok(true, "navigator.sync.unregister() oneShot done");
+      runTests();
+    }, genericError);
+  }
+
+  function test_unregister_multiShots() {
+    navigator.sync.unregister('multiShots').then(
+    function() {
+      ok(true, "navigator.sync.unregister() multiShots done");
+      runTests();
+    }, genericError);
+  }
+
   function test_wait() {
     // nothing to do here.
   }
@@ -113,6 +129,9 @@
     test_register_multiShots,
 
     test_wait,
+
+    test_unregister_oneShot,
+    test_unregister_multiShots,
   ];
 
   function runTests() {
diff --git a/dom/webidl/Navigator.webidl b/dom/webidl/Navigator.webidl
index 653facf8237..64b67d5c434 100644
--- a/dom/webidl/Navigator.webidl
+++ b/dom/webidl/Navigator.webidl
@@ -285,6 +285,12 @@ partial interface Navigator {
   void    mozSetMessageHandler (DOMString type, systemMessageCallback? callback);
   [Throws, Pref="dom.sysmsg.enabled"]
   boolean mozHasPendingMessage (DOMString type);
+
+  // This method can be called only from the systeMessageCallback function and
+  // it allows the page to set a promise to keep alive the app until the
+  // current operation is not fully completed.
+  [Throws, Pref="dom.sysmsg.enabled"]
+  void mozSetMessageHandlerPromise (Promise<any> promise);
 };
 
 #ifdef MOZ_B2G_RIL