diff -r 000000000000 -r 6474c204b198 browser/components/downloads/src/DownloadsStartup.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/browser/components/downloads/src/DownloadsStartup.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,80 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=2 et sw=2 tw=80 filetype=javascript: */
+/* 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 component enables the JavaScript API for downloads at startup.  This
+ * will eventually be removed when nsIDownloadManager will not be available
+ * anymore (bug 851471).
+ */
+
+"use strict";
+
+////////////////////////////////////////////////////////////////////////////////
+//// Globals
+
+const Cc = Components.classes;
+const Ci = Components.interfaces;
+const Cu = Components.utils;
+const Cr = Components.results;
+
+Cu.import("resource://gre/modules/XPCOMUtils.jsm");
+
+/**
+ * CID and Contract ID of our implementation of nsIDownloadManagerUI.
+ */
+const kDownloadsUICid = Components.ID("{4d99321e-d156-455b-81f7-e7aa2308134f}");
+const kDownloadsUIContractId = "@mozilla.org/download-manager-ui;1";
+
+/**
+ * CID and Contract ID of the JavaScript implementation of nsITransfer.
+ */
+const kTransferCid = Components.ID("{1b4c85df-cbdd-4bb6-b04e-613caece083c}");
+const kTransferContractId = "@mozilla.org/transfer;1";
+
+////////////////////////////////////////////////////////////////////////////////
+//// DownloadsStartup
+
+function DownloadsStartup() { }
+
+DownloadsStartup.prototype = {
+  classID: Components.ID("{49507fe5-2cee-4824-b6a3-e999150ce9b8}"),
+
+  _xpcom_factory: XPCOMUtils.generateSingletonFactory(DownloadsStartup),
+
+  //////////////////////////////////////////////////////////////////////////////
+  //// nsISupports
+
+  QueryInterface: XPCOMUtils.generateQI([Ci.nsIObserver]),
+
+  //////////////////////////////////////////////////////////////////////////////
+  //// nsIObserver
+
+  observe: function DS_observe(aSubject, aTopic, aData)
+  {
+    if (aTopic != "profile-after-change") {
+      Cu.reportError("Unexpected observer notification.");
+      return;
+    }
+
+    // Override Toolkit's nsIDownloadManagerUI implementation with our own.
+    // This must be done at application startup and not in the manifest to
+    // ensure that our implementation overrides the original one.
+    Components.manager.QueryInterface(Ci.nsIComponentRegistrar)
+                      .registerFactory(kDownloadsUICid, "",
+                                       kDownloadsUIContractId, null);
+
+    // Override Toolkit's nsITransfer implementation with the one from the
+    // JavaScript API for downloads.
+    Components.manager.QueryInterface(Ci.nsIComponentRegistrar)
+                      .registerFactory(kTransferCid, "",
+                                       kTransferContractId, null);
+  },
+};
+
+////////////////////////////////////////////////////////////////////////////////
+//// Module
+
+this.NSGetFactory = XPCOMUtils.generateNSGetFactory([DownloadsStartup]);