[devtools] "Keep" some temporary files/dirs around to help with debugging The whole QUnitTestCase runs with an @with_tempdir so it's redundant anyway.

[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)