I use Firefox.
But.... just so you guys know, don't believe the hype.
In most benchmarking tests, Firefox is NOT faster than the latest couple of versions of Internet Explorer. One difference is that it starts showing tables right away and then lets you watch as it shifts everything around to where it should be, while IE will wait until the whole thing is loaded to display. But it's just an illusion of speed.
Firefox is also a memory hog compared to IE. And they take longer to send out security patches than Microsoft. And Flash in FF is very slow compared to IE.
IE 8 which was release a few weeks back finally adheres to website programming standards, so if that site you were viewing in IE 7 was messed up, it would probably be fine in IE 8. By all accounts, IE 8 is a great browser.
If you want the fastest browser, Google Chrome is the way to go. They just release version 2.0. Supposedly way way faster than the others (including Safari, which uses the same underlying display technology). The Opera browser would be next in line.
I actually have all of these (IE8, Firefox, Chrome, Opera, Safari) installed in order to double-check websites I design, but I'm just used to using Firefox so that's what I've stuck with.