I have to say I'm very impressed with Mozilla Firefox.
Historically, I've always been an Internet Explorer user, but have recently HAD to use Firefox, when my IE browser would not support a Web-based video conference -- Firefox worked.
Once installed I played around with it and fell in love. It manages add-ons much better than IE and has a very healthy add-on community. What I especially love is the ability for Firefox to detect whether add-on updates are available and prompt you to perform the update immediately.
In terms of speed, I don't recall a significant difference - so I can't complain one way or another.
Now I'm curious how google chrome stacks up, but I guess i'll wait a while before trying it.