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.
Hmmm, I think it's computer speed. I use notebook with just a single core 1.7 GHz Pentium M.
What's interesting is that for me page scrolling speed isn't spread over many values for different sites. It's either fast (majority of sites) or very slow (some sites).
That's why I didn't suspect computer speed. It did look more like rendering engine bug / inefficient code path for particular layout/image/CSS combinations.
It seems like some designs trigger heavy computation, which is not noticeable on faster systems, but is very painful on slower machines.
