diff --git a/toolkit/mozapps/downloads/src/DownloadUtils.jsm b/toolkit/mozapps/downloads/src/DownloadUtils.jsm
index 05893ebfc984..12aaffb29876 100644
--- a/toolkit/mozapps/downloads/src/DownloadUtils.jsm
+++ b/toolkit/mozapps/downloads/src/DownloadUtils.jsm
@@ -140,6 +140,11 @@ let gStr = {
 // Initialize the lazy string getters!
 gStr._init();
 
+// Keep track of at most this many second/lastSec pairs so that multiple calls
+// to getTimeLeft produce the same time left
+const kCachedLastMaxSize = 10;
+let gCachedLast = [];
+
 let DownloadUtils = {
   /**
    * Generate a full status string for a download given its current progress,
@@ -248,6 +253,15 @@ let DownloadUtils = {
     if (aSeconds < 0)
       return [gStr.timeUnknown, aLastSec];
 
+    // Try to find a cached lastSec for the given second
+    aLastSec = gCachedLast.reduce(function(aResult, aItem)
+      aItem[0] == aSeconds ? aItem[1] : aResult, aLastSec);
+
+    // Add the current second/lastSec pair unless we have too many
+    gCachedLast.push([aSeconds, aLastSec]);
+    if (gCachedLast.length > kCachedLastMaxSize)
+      gCachedLast.shift();
+
     // Apply smoothing only if the new time isn't a huge change -- e.g., if the
     // new time is more than half the previous time; this is useful for
     // downloads that start/resume slowly
diff --git a/toolkit/mozapps/downloads/tests/unit/test_syncedDownloadUtils.js b/toolkit/mozapps/downloads/tests/unit/test_syncedDownloadUtils.js
new file mode 100644
index 000000000000..2edb41cf2338
--- /dev/null
+++ b/toolkit/mozapps/downloads/tests/unit/test_syncedDownloadUtils.js
@@ -0,0 +1,58 @@
+/* ***** BEGIN LICENSE BLOCK *****
+ * Version: MPL 1.1/GPL 2.0/LGPL 2.1
+ *
+ * The contents of this file are subject to the Mozilla Public License Version
+ * 1.1 (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ * http://www.mozilla.org/MPL/
+ *
+ * Software distributed under the License is distributed on an "AS IS" basis,
+ * WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
+ * for the specific language governing rights and limitations under the
+ * License.
+ *
+ * The Original Code is DownloadUtils Test Code.
+ *
+ * The Initial Developer of the Original Code is
+ * Edward Lee <edward.lee@engineering.uiuc.edu>.
+ * Portions created by the Initial Developer are Copyright (C) 2008
+ * the Initial Developer. All Rights Reserved.
+ *
+ * Contributor(s):
+ *
+ * Alternatively, the contents of this file may be used under the terms of
+ * either the GNU General Public License Version 2 or later (the "GPL"), or
+ * the GNU Lesser General Public License Version 2.1 or later (the "LGPL"),
+ * in which case the provisions of the GPL or the LGPL are applicable instead
+ * of those above. If you wish to allow use of your version of this file only
+ * under the terms of either the GPL or the LGPL, and not to allow others to
+ * use your version of this file under the terms of the MPL, indicate your
+ * decision by deleting the provisions above and replace them with the notice
+ * and other provisions required by the GPL or the LGPL. If you do not delete
+ * the provisions above, a recipient may use your version of this file under
+ * the terms of any one of the MPL, the GPL or the LGPL.
+ *
+ * ***** END LICENSE BLOCK ***** */
+
+/**
+ * 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);
+}