Firefox is dead to me. I switched to Chrome a while back when FF started slowing down, never tried any of the beta's since I had had it with the whole thing, but figured they would be done fixing any problems..
Alas, I'd have to say FF now holds the crown for slowest browser out there, even compared to IE9. Depressing really. And I have gone to some length to test it, including a clean install of Windows and tried it on my MBP, with the clean OSX install on a separate partition.. No avail.. Still slow.
To hell with them. I'd use anything before them now.