I'm currently running Ubuntu Hardy Heron and Firefox 3.0.4, but I dual boot and I've never had that problem on Windows Vista and Firefox 3.0.4 either. I don't think it matters, but my processor is an AMD Turion 64-bit dual core.

Maybe it's a difference in some algorithm between FF/Chrome and Explorer/Opera/Safari, though Chrome and Safari share at least most of their rendering code. That leads me to believe that it's probably something to do with Flash or something embedded. I don't know what to tell you, though, I've never had any problems, and my computer isn't even that fast.

What are your computers' specs?