If you're running these tests and you get an error like this: FAIL head.js import threw an exception - Error opening input stream (invalid filename?): chrome://mochitests/content/browser/toolkit/mozapps/update/tests/browser/head.js Then run `mach test toolkit/mozapps/update/tests/browser` first. You can stop mach as soon as it starts the first test, but this is necessary so that mach builds the update tests in your objdir.