Very seldom ever have problems with FF on my machine, and I've been running the Nightly alphas (presently at v13.0 Alpha 1) for a quite a few months now. Heck, even the infamous memory leaks haven't been very noticeable since back several releases -- leaving FF running for days at a time on my machine doesn't consume any more memory than does Chrome or IE9 with the same tabs open.
OP, have you tried to disable hardware acceleration in the Advanced Settings menu under Tools and Options? That is one feature I definitely see issues with from time to time (though more due to nVidia's video drivers crashing than anything else). I also agree with the advice about uninstalling and deleting your profile, then reinstalling. Corrupted FF profiles can cause all sorts of weird issues, and simply dropping a version upgrade on top of a corrupted profile won't solve anything.