I’ve had problems before where it would have been useful to easily run mochitests against other versions of the browser than those in my $OBJDIR. I’m pleased to say I just landed a small patch for the mochitests runner’s mach commands that lets you do:
./mach mochitest-browser --app-override /usr/bin/firefox path/to/test
and also has a special shortcut for the application as built for distribution (using omni.jar rather than a flat directory structure):
./mach mochitest-browser --app-override dist path/to/test
Happy testing!