Over 190 bugs has been fixed since beta 1, dramatically increasing the browsers performance.
A new feature has emerged in this beta, as a new mechanism to prevent incompatible software from crashing Firefox (specifically DLL files in Windows).
A full list of changes can be found in the release notes.
Users of Firefox 3.6 Beta 1 will automatically receive the update, while new beta testers can download Firefox 3.6 Beta 2 from the Firefox beta site.
Firefox 3.6 final is scheduled for November 26.