Other additions include single-click appearance modifications via Personas, notifications for outdated plugins, improved automatic form fill, and support for the WOFF font format, HTML 5 File API, CSS, and DOM. Web and add-on developers should swing by Mozilla's official developer page for more information about the changes.
It's noted that over 75% of existing add-ons are already compatible with Firefox 3.6, so you should be okay to jump the gun. If you run into any compatibility trouble though, we published a workaround last year, and a stable release is expected soon. For those already running a 3.6 beta, it should be updated automatically.