STEPS TO REPRODUCE: 1) Do a try run that fails in two different mochitests, A and B. 2) "mach mochitest A" to debug the issue with A. 3) Edit URL to point to B instead of A (to avoid the ridiculous time it takes to shut down and restart a mochitest run). EXPECTED RESULTS: Test B loads and I can debug it. ACTUAL RESULTS: Starting sometime not too long ago, the URL of B gives a 404. I suspect this might be a result of bug 1242051.

I think the thing to do here will be to provide an option to disable bug 1242051, and always install all the tests. If we can't tell from the command line passed which tests are going to be run, that approach really isn't going to work.