I just installed it but haven't use it yet. I've noticed issues with their latest updates, it's like they're trying too hard; kind of like facebook(won't get into that though). I do have quite a few add ons and plug ins on there too(some plug ins haven't been working since 5.0).
Something else I recently noticed was my CPU usage shooting up while using firefox(the newest update before the new 7... Usually it'll run around 2-5% usage but when I open up firefox it shoots to 20%. So if that's happening to me, it may be why yours is locking up on you.
its a gaming computer, but i guess since its ~3 yrs old... >.>