The guys from HowToCreate.co.uk did some extensive browser testing. Check out the results here. You will find a bunch of numbers and graphs, which may or may not be helpful to you. They did not benchmark plugins like Java or Flash, which is of great importance in browser speed comparisons. Conclusion of the article is that Opera is fastest browser on PC. Well, from my personal experience, I can agree with that. My Firefox 2.0 performs very badly after installation of 20 3rd party extensions. But I love those extensions, and without them, my day to day developing would be a lot of pain. :)
Cmon guys, test Flash plugin performance!