I did a test with the new Firefox 3 beta 3 last weekend. It's the first 3.x release I had a look at and I think it's stable enough for immediate use.
What I like most is the new download manager implementation. The old one was quite horrible. You can tell Firefox to reload the tabs from the last session now. It seems to be a bit faster, too.
It takes all configurations from your previous Firefox installation and tries to replace your plugins. For Firebug it doesn't find a replacement automatically. But, the Firebug author already offers a 1.1 beta release you can install with a click. Have a look at getfirebug.com/releases
If you don't know what Firebug is for, here are the details. It's one of the coolest plugins for Firefox these days ;-).