Mozilla released a second version of the Firefox 3.6 beta yesterday, and issued an update for the first Firefox 3.6 beta. The new build contains over 190 fixes from the original beta, along with several improvements for web developers, add-on developers, and users.
Mozilla notes that developers can read more about the new features in Firefox 3.6 on the Mozilla Developer Center. You can download the second Firefox 3.6 beta here: Windows, Mac OS X, Linux. Remember that many add-ons may not work with 3.6 yet, but you might have luck with this workaround.