IE9 ditched XP because much of that fancy GPU accelerated rendering cannot be done (well?) in XP. I've read that while FF4 works in XP, it falls back to software rendering, so it's not that impressive anyway.
http://arstechnica.com/microsoft/review ... viewed.ars
I tried it and it seems nice. I'm still more used to the FF interface, so I'll probably keep using that.