Started yesterday, I think. Happens when i'm watching videos on youtube, or even on regular websites. All of a sudden, the screen goes black, firefox crashes, and I get a "send error report" from microsoft and mozilla. Point me in the right direction, please.
You mention youtube, Flash comes to mind. Is it up to date?
The other possibility, which is overly tedious, is to start disabling your extensions one at a time until you've found a problem. But who has time for that?
I bet if you create a new firefox profile, the problem will go away.
Hope it helps.
Edit: you don't mention if FF is up to date, but I think there was a bad release early on in 32 or 33. Just make sure you are running the latest and greatest. The FF development train is moving quite fast!