Well FF & Chrome act the same way on my laptop, with a few tabs open, a few flash video's here and there and it's GUI lockup HARD time. No way to even launch xkill or a terminal window, hard drive access is like crazy when this happens.
I get fed up enough to just power it off abruptly! (Less stressful when the screen goes dark
Took the HD out, put a fresh copy of XFCE on an old 60gb SATA drive, got everything working OK, no lockups (yet) with FF!!!
I'm thinking KDE just doesn't play well with my hardware or something but I have no compunction about wiping the KDE install off my original drive & doing an XFCE install if further testing goes smoothly.
JMHO, YMMV. ;-)