diff -r 000000000000 -r 6474c204b198 toolkit/components/search/tests/xpcshell/test_prefSync.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/toolkit/components/search/tests/xpcshell/test_prefSync.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,130 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+/*
+ * Test that currentEngine and defaultEngine properties are updated when the
+ * prefs are set independently.
+ */
+
+"use strict";
+
+const { classes: Cc, interfaces: Ci, utils: Cu, results: Cr } = Components;
+
+Cu.import("resource://testing-common/httpd.js");
+
+let waitForEngines = {
+  "Test search engine": 1,
+  "A second test engine": 1
+};
+
+const PREF_BRANCH = "browser.search.";
+
+/**
+ * Wrapper for nsIPrefBranch::setComplexValue.
+ * @param aPrefName
+ *        The name of the pref to set.
+ */
+function setLocalizedPref(aPrefName, aValue) {
+  let nsIPLS = Ci.nsIPrefLocalizedString;
+  let branch = Services.prefs.getBranch(PREF_BRANCH);
+  try {
+    var pls = Cc["@mozilla.org/pref-localizedstring;1"].
+              createInstance(Ci.nsIPrefLocalizedString);
+    pls.data = aValue;
+    branch.setComplexValue(aPrefName, nsIPLS, pls);
+  } catch (ex) {}
+}
+
+function search_observer(aSubject, aTopic, aData) {
+  let engine = aSubject.QueryInterface(Ci.nsISearchEngine);
+  do_print("Observer: " + aData + " for " + engine.name);
+
+  if (aData != "engine-added") {
+    return;
+  }
+
+  // If the engine is defined in `waitForEngines`, remove it from the list
+  if (waitForEngines[engine.name]) {
+    delete waitForEngines[engine.name];
+  } else {
+    // This engine is not one we're waiting for, so bail out early.
+    return;
+  }
+
+  // Only continue when both engines have been loaded.
+  if (Object.keys(waitForEngines).length) {
+    return;
+  }
+
+  let search = Services.search;
+
+  let engine1Name = "Test search engine";
+  let engine2Name = "A second test engine";
+  let engine1 = search.getEngineByName(engine1Name);
+  let engine2 = search.getEngineByName(engine2Name);
+
+  // Initial sanity check:
+  search.defaultEngine = engine1;
+  do_check_eq(search.defaultEngine, engine1);
+  search.currentEngine = engine1;
+  do_check_eq(search.currentEngine, engine1);
+
+  setLocalizedPref("defaultenginename", engine2Name);
+  // Default engine should be synced with the pref
+  do_check_eq(search.defaultEngine, engine2);
+  // Current engine should've stayed the same
+  do_check_eq(search.currentEngine, engine1);
+  
+  setLocalizedPref("selectedEngine", engine2Name);
+  // Default engine should've stayed the same
+  do_check_eq(search.defaultEngine, engine2);
+  // Current engine should've been updated
+  do_check_eq(search.currentEngine, engine2);
+
+  // Test that setting the currentEngine to the original default engine clears
+  // the selectedEngine pref, rather than setting it. To do this we need to
+  // set the value of defaultenginename on the default branch.
+  let defaultBranch = Services.prefs.getDefaultBranch("");
+  let prefName = PREF_BRANCH + "defaultenginename";
+  let prefVal = "data:text/plain," + prefName + "=" + engine1Name;
+  defaultBranch.setCharPref(prefName, prefVal, true);
+  search.currentEngine = engine1;
+  // Current engine should've been updated
+  do_check_eq(search.currentEngine, engine1);
+  do_check_false(Services.prefs.prefHasUserValue("browser.search.selectedEngine"));
+
+  // Test that setting the defaultEngine to the original default engine clears
+  // the defaultenginename pref, rather than setting it.
+  do_check_true(Services.prefs.prefHasUserValue("browser.search.defaultenginename"));
+  search.defaultEngine = engine1;
+  do_check_eq(search.defaultEngine, engine1);
+  do_check_false(Services.prefs.prefHasUserValue("browser.search.defaultenginename"));
+
+  do_test_finished();
+}
+
+function run_test() {
+  removeMetadata();
+  updateAppInfo();
+
+  let httpServer = new HttpServer();
+  httpServer.start(-1);
+  httpServer.registerDirectory("/", do_get_cwd());
+  let baseUrl = "http://localhost:" + httpServer.identity.primaryPort;
+
+  do_register_cleanup(function cleanup() {
+    httpServer.stop(function() {});
+    Services.obs.removeObserver(search_observer, "browser-search-engine-modified");
+  });
+
+  do_test_pending();
+
+  Services.obs.addObserver(search_observer, "browser-search-engine-modified", false);
+
+  Services.search.addEngine(baseUrl + "/data/engine.xml",
+                   Ci.nsISearchEngine.DATA_XML,
+                   null, false);
+  Services.search.addEngine(baseUrl + "/data/engine2.xml",
+                   Ci.nsISearchEngine.DATA_XML,
+                   null, false);
+}