This documentation discusses how to write a test for the address bar and the different test utilities that are useful when writing a test for the address bar.

Common Tests


Some common tests for the address bar are the mochitests. The purpose of a mochitest is to run the browser itself. Mochitests can be called “browser tests”, “mochitest-browser-chrome”, or “browser-chrome-mochitests”. There are other types of mochitests that are not for testing the browser and therefore can be ignored for the purpose of the address bar. An example of a mochitest is tests/browser/browser_switchTab_currentTab.js


XPCShell Tests are another type of test relevant to the address bar. XPCShell tests are often called unit tests because they tend to test specific modules or components in isolation, as opposed the mochitest which have access to the full browser chrome.

XPCShell tests do not use the browser UI and are completely separate from browser chrome. XPCShell tests are executed in a JavaScript shell that is outside of the browser. For historical context, the “XPC” naming convention is from XPCOM (Cross Platform Component Model) which is an older framework that allows programmers to write custom functions in one language, such as C++, and connect it to other components in another language, such as JavaScript.

Each XPCShell test is executed in a new shell instance, therefore you will see several Firefox icons pop up and close when XPCShell tests are executing. These are two examples of XPCShell tests for the address bar test_providerHeuristicFallback and test_providerTabToSearch.

When To Write a XPCShell or Mochitest?

Always default to writing an XPCShell test if it is possible. XPCShell tests are faster to execute than browser tests. Although, most of the time you will write a browser test because you could be modifying something in the UI or testing a specific component in the UI.

If you are writing a test for a urlbarProvider, you can test the Provider through a XPCShell test. Providers do not modify the UI, instead what they do is receive a url string query, search for the string and bring back the result. An example is the ProviderPlaces, which fetches results from the Places database. Another component that’s good for writing XPCShell test is the urlbarMuxer.

There may be times where writing both an XPCShell test and browser test is necessary. In these situations, you could be testing the result from a Provider and also testing what appears in the UI is correct.

How To Write a Test

Test Boilerplate

This basic test boilerplate includes a license code at the top and this license code is present at the top of every test file, the "use strict" string is to enable strict mode in JavaScript, and add_task function adds tests to be executed by the test harness.

/* Any copyright is dedicated to the Public Domain.
 * http://creativecommons.org/publicdomain/zero/1.0/ */

 * This tests ensures that the urlbar ...

"use strict";

add_task(async function testOne() {
  // testing code and assertions

add_task(async function testTwo() {
  // testing code and assertions

In order to run a test use the ./mach command, for example, ./mach test <path to test file> to run test locally. Use the command with --jsdebugger argument at the end to open the DevTools debugger to step through the test, ./mach test <path to test> --jsdebugger.


The manifest’s purpose is to list all the test in the directory and dictate to the test harness which files are test and how the test harness should run these test. Anytime a test is created, the test file name needs to be added to the manifest in alphabetical order.

Start in the manifest file and add your test name in alphabetical order. The manifest file we should add our test in is browser.ini. The urlbar/test/browser/ directory is the main browser test directory for address bar, and the manifest file linked above is the main browser test manifest. The .ini file extension is an initialization file for Windows or MS-DOS.

Manifest Metadata

The manifest file can define common keys/metadata to influence the test’s behavior. For example, the metadata support-files are a list of additional files required to run a test. Any values assigned to the key support-files only applies to the single file directly above the support-files key. If more files require support-files, then support-files need to be added directly under the other test file names. Another example of a manifest metadata is [DEFAULT]. Anything under [DEFAULT] will be picked up by all tests in the manifest file.

For information on all the manifest metadata available, please visit Test Manifests.

Common Test Utilities

This section describes common test utilities which may be useful when writing a test for the address bar. Below are a description of common utils where you can find helpful testing methods.

Many test utils modules end with TestUtils.sys.mjs. However not every testing function will end with TestUtils.sys.mjs. For example, PlacesUtils does not have “Test” within its name.

A critical function to remember is the registerCleanupFunction within the head.js file mentioned below. This function’s purpose may be to clean up the history or any other clean ups that are necessary after your test is complete. Cleaning up after a browser test is necessary because clean up ensures what is done within one test will not affect subsequent tests.

head.js and common-head.js

The head.js file is executed at the beginning before each test and contains imports to modules which are useful for each test. Any tasks head.js adds (via add_task) will run first for each test, and any variables and functions it defines will be available in the scope of each test. This file is small because most of our Utils are actually in other .sys.mjs files.

The XPCOMUtils.defineLazyModuleGetters method within head.js sets up modules names to where they can be found, their paths. Lazy means the files are only imported if or when it is used. Any tests in this directory can use these modules without importing it themselves in their own file. The head.js provides a convenience for this purpose. The head.js file imports common-head.js making everything within head-common.js available in head.js as well.

The registerCleanupFunction is an important function in browser mochi tests and it is part of the test harness. This function registers a callback function to be executed when your test is complete. The purpose may be to clean up the history or any other clean ups that are necessary after your test is complete. For example, browser mochi tests are executed one after the other in the same window instance. The global object in each test is the browser window object, for example, each test script runs in the browser window. If the history is not cleaned up it will remain and may affect subsequent browser tests. For most test outside of address bar, you may not need to clear history. In addition to cleanup, head.js calls the registerCleanupFunction to ensure the urlbar panel is closed after each test.


UrlbarTestUtils.sys.mjs is useful for url bar testing. This file contains methods that can help with starting a new search in the url bar, waiting for a new search to complete, returning the results in the view, and etc.


BrowserTestUtils.sys.mjs is useful for browser window testing. This file contains methods that can help with opening tabs, waiting for certain events to happen in the window, opening new or private windows, and etc.


TestUtils.sys.mjs is useful for general purpose testing and does not depend on the browser window. This file contains methods that are useful when waiting for a condition to return true, waiting for a specific preference to change, and etc.


PlacesTestUtils.sys.mjs is useful for adding visits, adding bookmarks, waiting for notification of visited pages, and etc.


EventUtils.js is an older test file and does not need to be imported because it is not a .sys.mjs file. EventUtils is only used for browser tests, unlike the other TestUtils listed above which are used for browser tests, XPCShell tests and other tests.

All the functions within EventUtils.js are automatically available in browser tests. This file contains functions that are useful for synthesizing mouse clicks and keypresses. Some commonly used functions are synthesizeMouseAtCenter which places the mouse at the center of the DOM element and synthesizeKey which can be used to navigate the view and start a search by using keydown and keyenter arguments.