I haven't used IE8 much, don't have it on a PC here, but...
I have used Firefox for a few years, and been very satisfied. Since I starting using 3.x, however, I get frequent crashes on a few sites.
Ex, it will crash in Yahoo mail classic once in a while. No big deal unless I lose a time-consuming composition.
But, FF 3.x (from 3.0.1 to 3.0.11 I am using now) crash about every other time I open iGoogle. I keep thinking the next 3.0.x will fix it, but it doesn't.
I'm sticking with Firefox, but this crash can be annoying. I must remember not to open iGoogle when I am working on a process in another FF window. The recovery feature is great, but doesn't always work as hoped due to restrictions imposed by the web site.
BTW, I think one reason IE8 can be so slow due to the checks it attempts for every site you visit.