OK, so I got mine yesterday direct from an FTP server, but finally, it’s officially available for download. Go get!
From an end user perspective nothing major has changed, rather the whole package looks, feels, and behaves more polished. Underneath though it’s tighter, sharper, more secure and even more useful. The best ‘new thing’ for me is the in-line spell checking, though I had to go manually download the British English Dictionary Add-on in order for it to work. May I also suggest you try Fission, which is an add-on that enhances the address bar by showing loading progress, and a few other things if you look into the add-on options once you have it installed.
Be sure to investigate the Tools > Options section, and have a play. Built-in session-recover behaviour is a beautiful thing. Now Firefox opens exactly where I left it, even after shutting it down. (OK, the Tab Mix Plus extension did this before, but it’s now integrated.)