[devtools] Fix Firefox launcher in QUnitTestCase (closes #4294727)
The main changes are:
- stop creating the profile, firefox will create it
- point firefox to a profile directory instead of giving it a profile name
(this has the added bonus of not polluting the user's profile list)
- start firefox once and kill it 5 seconds later to let it finish its profile
creation (along with system-wide extensions setup)