diff -r 000000000000 -r 6474c204b198 modules/libpref/test/unit/head_libPrefs.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/modules/libpref/test/unit/head_libPrefs.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,45 @@
+/* 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/. */
+
+const NS_APP_USER_PROFILE_50_DIR = "ProfD";
+
+const Ci = Components.interfaces;
+const Cc = Components.classes;
+const Cr = Components.results;
+const Cu = Components.utils;
+
+function do_check_throws(f, result, stack)
+{
+  if (!stack)
+    stack = Components.stack.caller;
+
+  try {
+    f();
+  } catch (exc) {
+    if (exc.result == result)
+      return;
+    do_throw("expected result " + result + ", caught " + exc, stack);
+  }
+  do_throw("expected result " + result + ", none thrown", stack);
+}
+
+var dirSvc = Cc["@mozilla.org/file/directory_service;1"].getService(Ci.nsIProperties);
+
+// Register current test directory as provider for the profile directory.
+var provider = {
+  getFile: function(prop, persistent) {
+    persistent.value = true;
+    if (prop == NS_APP_USER_PROFILE_50_DIR)
+      return dirSvc.get("CurProcD", Ci.nsIFile);
+    throw Cr.NS_ERROR_FAILURE;
+  },
+  QueryInterface: function(iid) {
+    if (iid.equals(Ci.nsIDirectoryServiceProvider) ||
+        iid.equals(Ci.nsISupports)) {
+      return this;
+    }
+    throw Cr.NS_ERROR_NO_INTERFACE;
+  }
+};
+dirSvc.QueryInterface(Ci.nsIDirectoryService).registerProvider(provider);