had a similar problem (and I was the one, who has reported it in bugs).
even with cash clearing and such, firefox remained sluggish at best (deleting cache, cookies, flash cache, etc did not help fully), and even more, even the german accounts, which were never targeted by this update, showed problems (CPU load with a 4x 3ghz CPU at 70-80%).
switched to the vivaldi browser, and I have to say, it works better than ever.
(vivaldi, made by the devs, who weren't happy about how the opera development progressed and did something else... it's worth a look)