Firefox 4.0 and Adobe Flash Player 10 are not compatible.
I upgraded to Firefox 4.0 and immediately had trouble with it repeatedly hanging/not responding. After going through the add ons and plugins I discovered that the culprit is the Adobe Flash Player. I had just upgraded that as well. It works perfectly fine in Internet Explorer but I have to disable it in Firefox in order to use the browser. I am unable to watch videos, etc. What's the solution for this? Both Firefox and Adobe Flash are up to date. They just aren't compatible.
I've never had problems with Firefox until I upgraded to 4.0. Personally, I'd like to revert to my old version [3.6]. At least until an actual, working upgrade is available.