Request a mochitest loaner on Try, ran run-wizard with 2 (setup). Also fails with the mozilla repository checked out and referenced. root@d597d514cef5:~# mach mochitest browser/components/sessionstore/test/browser_625016.js Error running mach: ['mochitest', 'browser/components/sessionstore/test/browser_625016.js'] The error occurred in mach itself. This is likely a bug in mach itself or a fundamental problem with a loaded module. Please consider filing a bug against mach by going to the URL: https://bugzilla.mozilla.org/enter_bug.cgi?product=Firefox%20Build%20System&component=Mach%20Core If filing a bug, please include the full output of mach, including this error message. The details of the failure are as follows: ImportError: No module named mozbuild.base File "/builds/worker/workspace/build/tests/tools/mach/mach/main.py", line 359, in run return self._run(argv) File "/builds/worker/workspace/build/tests/tools/mach/mach/main.py", line 506, in _run from mozbuild.base import BuildEnvironmentNotFoundException