I've been using firefox exclusively for about a year now. It's awesome. It's the fastest most stable browser I've ever used (yes, I've used opera). The GUI is a good mix between IE and Opera, and it renders faster than both. The pop-up blocker rules, and I can't get exploited through a web page (thanks for nothing M$). I haven't seen a pop-up in a year with FireFox. Also, CTRL-T becomes your best friend.
For testing web pages I usually use opera. It tends to break pretty easily because it doesn't make any assumptions about bad html coding.
-tom
