I was using Firefox 3 Beta 3 for a bit when it came out … last month? I really like the interface and features, and seemed pretty stable, but it was failing utterly to render some pages, so I went back to Camino. Just noticed Beta 4 is out, so I’ll be trying this again for a while. You’ll have to dig around on your own for the URL to download it. Mine just auto-updated from Beta 3. 🙂
Also, I have to give some credit to Netvibes for making browser switching so easy. No need to worry about copying over your bookmarks, etc. They come with you no matter what browser you use.