Firefox 3.0 has been released. I really like it and would suggest it to anyone that is still using Internet Explorer, or the older version. It's completely free to download. http://www.mozilla.com/en-US/firefox/
The two are pretty similar if I recall correctly. Been awhile since I used Opera, and even then I didn't use it much.
I haven't used it myself, but my daughter had it on hers. She has since switched to Opera. Firefox I found was an improvement over IE but still prone to flaws. Opera has been trouble free for me for the three years I've been using it, plugins and codecs are included and the browser is completely customizable. A bit of a learning curve for a couple of days but the more you play with it and get it the way you want it, you'll wonder why you didn't years ago. I highly recommend it for anyone wanting trouble free browsing and being able to do it on their terms.