$ mach mochitest-plain Error running mach: ['mochitest-plain'] The error occurred in the implementation of the invoked mach command. This should never occur and is likely a bug in the implementation of that command. Consider filing a bug for this issue. If filing a bug, please include the full output of mach, including this error message. The details of the failure are as follows: TypeError: __init__() takes at most 2 arguments (3 given) File "/src/mozilla-central/testing/mochitest/mach_commands.py", line 268, in run_mochitest_plain return self.run_mochitest(test_file, 'plain', **kwargs) File "/src/mozilla-central/testing/mochitest/mach_commands.py", line 299, in run_mochitest **kwargs) File "/src/mozilla-central/testing/mochitest/mach_commands.py", line 111, in run_mochitest_test opts = mochitest.MochitestOptions(automation, tests_dir)