Currently debugging Mochitests means that you have to run runtests.py, then attach a debugger, then run the test you want to run. It would be nice if you could just say `runtests.py --debug`, and runtests.py would start the browser in a debugger for you. This would make it very easy to debug a test by doing `runtests.py --debug --autorun --test-path=foo`. In addition, we should probably provide an arbitrary --run-with command for things like valgrind.