Firefox is pretty slick...
I'm using it right now
It'* still in the pre 1.0 stages but the tabbed browsing and no popups make it pretty slick.
Version 0.9 was released just a week or two ago, I think...
The only down side that I've run into is having to install plugins, which isn't a big deal, but may be a bit intimidating for people who aren't tech savy.
I still keep IE on my machine, and regret to say that I use it from time to time, IE has been around for years and years where Firefox is pretty much brand new, so there are certian features that aren't there yet... (but I'll be they'll be there in 1.0)
Anyway, check it out if you haven't already.