I know it's not super believable, but when I'm at work I have two or three IE pages open and maybe a firefox page, all with 20+ tabs open (that's when I have to start scrolling through my tabs.. so I open another IE.) And I do some website editing so I can test in FF and IE...
It takes an insane amount of memory! The computer will start acting up eventually so I'll just close all of the IEs and FFs and it'll be good again
I think you're probably right when you say you thing FF was the problem... especially if you have a lot of tabs open