That Firefox 3.6 isn't polished yet is a given. That we're seeing a new beta release so soon is something else. Destined to be released this year Mozilla cranked up a new beta to fix the most important bugs on the road to release candidates.

In this release alone 190 bugs were squashed, while the feature set remained the same compared with the last beta. Of course a new beta does not need new features if your goal is to stabilize it. For Mac users the most prominent fix is the lost of focus bug that plagues Firefox 3.5 in combination with Growl 1.2 (the only release that works on Snow Leopard). However for the rest of the world a lot of top crashers has been fixed, and well it's so much that you'll just have to dig through the bug list yourself.

Firefox 3.6 Beta users will get the update automatically (or through Help > Check for Updates...). If you want you can get the full download from Mozilla's web site.