BTW: I've recently read quite a few reports of major problems with 6.0
I'm unsure why this should be as there weren't that many "big" changes from version 5 to 6.
From doing a quick web search it seems some WIndows users are having issues which seem to point to their AV software.
My version of Firefox 6 is rock solid - hasn't given me any issues whatsoever.
I would update my system and mebbe install/reinstall xulrunner
And as mentioned try disabling Flash Hardware Acceleration
(is it only Flash sites that crash?)