diff -r 000000000000 -r 6474c204b198 toolkit/mozapps/downloads/tests/unit/test_syncedDownloadUtils.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/toolkit/mozapps/downloads/tests/unit/test_syncedDownloadUtils.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,26 @@
+/* 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/. */
+
+/**
+ * Test bug 420482 by making sure multiple consumers of DownloadUtils gets the
+ * same time remaining time if they provide the same time left but a different
+ * "last time".
+ */
+
+let Cu = Components.utils;
+Cu.import("resource://gre/modules/DownloadUtils.jsm");
+
+function run_test()
+{
+  // Simulate having multiple downloads requesting time left
+  let downloadTimes = {};
+  for each (let time in [1, 30, 60, 3456, 9999])
+    downloadTimes[time] = DownloadUtils.getTimeLeft(time)[0];
+
+  // Pretend we're a download status bar also asking for a time left, but we're
+  // using a different "last sec". We need to make sure we get the same time.
+  let lastSec = 314;
+  for (let [time, text] in Iterator(downloadTimes))
+    do_check_eq(DownloadUtils.getTimeLeft(time, lastSec)[0], text);
+}