I use dual monitors; although Firefox 3 worked fine, Firefox 4 will not work on monitor 2.
When I move Firefox 4 to monitor 2, and try to maximize the browser, it does not fully maximize. It also completely locks up, and the only things that work are the minimize and close buttons. Even if I don't maximize the browser, Firefox 4 still doesn't do anything on monitor 2.
Opaite Mbohovái (2)
Go to Options -> Advanced -> General and disable "Hardware Acceleration". I had the same issue and this helped!
Also take a look at https://bugzilla.mozilla.org/show_bug.cgi?id=632354
I also had similar problems. Had to do two things. Disable hardware acceleration as stated above, and disable the FoxTab add-on. So far, this seems to have solved the problems.