By Mozilla released the rc1 version of Firefox 3 last weekend. It's a lot more stable than the Beta versions and is significantly quicker at rendering web pages than Firefox 2 ever was. I've been using it side by side with Firefox 2 on my main PC since Sunday evening and although not all of my existing extensions work, I'm happy enough with it.
It is available for Windows, Mac OS X and Linux from here.
With the fact that I do a lot of JavaScript work, I need the FireBug plugin to work, so I won't be using it until I have seen that it works. What I have seen of it, it's really good.