Two of the biggest fixes posted included an issue with Firefox crashing due to interaction with certain plugins, along with the ability for the browser to execute scripts asynchronously. The latter, Mozilla claims, will bring a speed boost in loading certain pages.
A comprehensive list of all the fixes since the last beta is available here. Many of the bugs were crashes, a few were related to the user interface and other aesthetics, while others were related to mechanics like window focus. If you're already using a 3.6 beta, now's a good time to upgrade.