diff -r 000000000000 -r 6474c204b198 toolkit/components/downloads/test/unit/test_guid.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/toolkit/components/downloads/test/unit/test_guid.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,32 @@
+/* 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 dm = Cc["@mozilla.org/download-manager;1"].getService(Ci.nsIDownloadManager);
+
+function run_test()
+{
+  if (oldDownloadManagerDisabled()) {
+    return;
+  }
+
+  let server = new HttpServer();
+  server.start(-1);
+  let dl = addDownload(server);
+  do_test_pending();
+
+  do_print(dl.guid);
+  do_check_true(/^[a-zA-Z0-9\-_]{12}$/.test(dl.guid));
+
+  dm.getDownloadByGUID(dl.guid, function(status, result) {
+    do_check_eq(dl, result, "should get back some download as requested");
+    dl.cancel();
+
+    dm.getDownloadByGUID("nonexistent", function(status, result) {
+      do_check_eq(result, null, "should get back no download");
+      do_check_eq(Components.results.NS_ERROR_NOT_AVAILABLE, status,
+                  "should pass NS_ERROR_NOT_AVAILABLE on failure");
+      do_test_finished();
+    });
+  });
+}