I tried to create a new subtype of DevToolsLoader that would allow mocking services. This way, more tests could be written as xpcshell tests rather than mochitests. See the attached patch. This approach fails because, we think, the DevToolsLoader binds "require" to its own "this", which happens too early. This results on a call being made on an object that doesn't have _mockedServices set, leading to this trace: 0:00.09 LOG: Thread-1 ERROR TypeError: invalid 'in' operand this._mockedServices at /home/tromey/firefox-git/gecko-dev/obj-x86_64-unknown-linux-gnu/_tests/xpcshell/toolkit/devtools/tests/unit/head_devtools.js:33 DevToolsTestLoader.prototype.lazyServiceGetter@/home/tromey/firefox-git/gecko-dev/obj-x86_64-unknown-linux-gnu/_tests/xpcshell/toolkit/devtools/tests/unit/head_devtools.js:33:1 @resource://gre/modules/devtools/css-color.js:444:1 DevToolsLoader.prototype.require@resource://gre/modules/devtools/Loader.jsm:302:12 @/home/tromey/firefox-git/gecko-dev/obj-x86_64-unknown-linux-gnu/_tests/xpcshell/toolkit/devtools/tests/unit/test_cssColor.js:35:22 load_file@/home/tromey/firefox-git/gecko-dev/testing/xpcshell/head.js:600:7 _load_files@/home/tromey/firefox-git/gecko-dev/testing/xpcshell/head.js:612:3 _execute_test@/home/tromey/firefox-git/gecko-dev/testing/xpcshell/head.js:494:3 It would be nice if the loader supported this sort of mocking or subtyping.