A new feature is "Personas", which is nothing more than many themes for Firefox. Other feature implemented in Firefox 3.6 is a mechanism which will check for newer updates for your plug-ins, in order to keep you updated and make the navigation as safe as possible. Mozilla also claims that their browser is now 25% faster than the previous version, which makes it the fastest browser (any company says that when they launch a new version). But there is nothing surprising, that could attract me in order to leave Chrome...
If you want to see more (and video): http://www.mozilla.com/en-US/firefox/comingsoon/ (I don't know what is with that video, but it runs like the hell, even on Firefox 3.5.7)
Download link: http://www.mozilla.com/en-US/firefox/all-beta.html
Update: Download link for 3.6 final version: http://www.mozilla.com/en-US/firefox/firefox.html