To run the tests in this directory, from the top source directory, either invoke the test despatcher in mach: % ./mach test remote/marionette/test/xpcshell Or call out the harness specifically: % ./mach xpcshell-test remote/marionette/test/xpcshell The latter gives you the --sequential option which can be useful when debugging to prevent tests from running in parallel. When adding new tests you must make sure they are listed in xpcshell.ini, otherwise they will not run on try. See also ../../doc/Testing.md for more advice on our other types of tests.