Firefox 3.5 is out but if you're a Ubuntu user you have a long wait in front of you because the official package will only be available in karmic koala.

This article (in portuguese) will explain you how to get Firefox 3.5 installed side by side with your "official" one from the repos.

The only thing is that you'll have Firefox named Shiretoko (the official codename of the development version). You´ll find it (in gnome) under the "Internet" menu.

Happy browsing :)