I’m a recent convert to Firefox 3, since 3.0.5 to the newest 3.0.7. I will say it is a bunch faster than IE7 and IE8. In Windows 7 IE8 does seem quicker than it does under Vista Sp1 or SP2 RC1 or XP SP3. Firefox 3 is also quicker in Win7 and I’d say it is neck and neck with IE8. Only problem is my version of IE8 crashed in Win7 so I have been using FF exclusively until Microsoft offers a stand alone download of IE 8 for Win7 or if there is a way to repair the version I have now that is inoperable.
I'm pretty sure that MS acknowledges that IE8 does not yet run stably in Win7. I'm sure they'll address that quickly, but I haven't seen it yet.
Build 7057 includes the release version of IE8, btw.
I basic switch between all three, depending on what I'm doing.