diff -r 000000000000 -r 6474c204b198 modules/libpref/test/unit/test_bug577950.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/modules/libpref/test/unit/test_bug577950.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,28 @@
+/* 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/. */
+
+function run_test() {
+  var ps = Cc["@mozilla.org/preferences-service;1"].
+            getService(Ci.nsIPrefService);
+
+  var pb = Cc["@mozilla.org/preferences-service;1"].
+            getService(Ci.nsIPrefBranch);
+
+  var observer = {
+    QueryInterface: function QueryInterface(aIID) {
+      if (aIID.equals(Ci.nsIObserver) ||
+          aIID.equals(Ci.nsISupports))
+         return this;
+      throw Components.results.NS_NOINTERFACE;
+    },
+
+    observe: function observe(aSubject, aTopic, aState) {
+      // Don't do anything.
+    }
+  }
+
+  /* Set the same pref twice.  This shouldn't leak. */
+  pb.addObserver("UserPref.nonexistent.setIntPref", observer, false);
+  pb.addObserver("UserPref.nonexistent.setIntPref", observer, false);
+}