Fix Firefox's Full Screen Permissions Prompt

It looks like it may be slightly tricky but not impossible to fix the full screen permissions prompt. The full screen code lives in nsDocument::RequestFullScreen(). It actually registers an observer ("fullscreen-approved") topic for reacting to the prompt, but then goes ahead and reparents the element and full screens anyway.

We might be able to refactor this code such that it gets called from the observer callback, after the user has interacted with the dialog.

Probably best done after FF31-ESR though.