Mozilla has pushed out the first release candidate (RC1) of Firefox version 3.6 for Windows, Mac OS X and Linux. The jump from Firefox 3.5.7 (the latest stable build) to 3.6 brings many changes, including a new Gecko rendering engine, a quicker start-up, and improved browser responsiveness.
Other additions include single-click appearance modifications via Personas, notifications for outdated plugins, improved automatic form fill, and support for the WOFF font format, HTML 5 File API, CSS, and DOM.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.