I have been having the same problem as you, so you're not alone. I was using FF 3.1b1, so, thinking it was an issue with that version of FF, I uninstalled it and rolled back to a previous version.. 3.0.3; unfortunately though I'm still having the problems.
But, since I already have a thread going, I'll get to it when my first issue has been resolved.
Sorry I couldn't be of any help, just wanted to let you know you're not alone.
Just thought I'd mention, what I do is clear out my cache when this problem arises, and it fixes the problem, but only temporarily. Usually within an hour or two, I'll get the same problem- no css elements applied to the site what so ever; everything out of alignment.. so it's temporary fix, but by no means permanent. Hopefully somebody else will have a better answer.
Edited by spyce, 30 October 2008 - 03:49 PM.