The Tor Browser / file revision

 A word to the wise:

 You must ensure that if your test finishes successfully, no processes have

 priority FOREGROUND_HIGH.

 If you don't do this, expect to see tests randomly fail with mysterious

 FOREGROUND --> FOREGROUND priority transitions.

 What's happening in this case is that your FOREGROUND_HIGH process lives until

 the beginning of the next test.  This causes the process started by the next

 test to have low CPU priority.  Then the FOREGROUND_HIGH process dies, because

 its iframe gets GC'ed, and we transition the new test's process from low CPU

 priority to regular CPU priority.

 Ouch.