Just been playing around with the RC2 of Firefox and gotta say its looking kinda nice. One thing I did find when downloading the browser is a complete list of releases from 0.8 right up to RC2 (as of today). Incase you ever need, or want to go back you can.
http://ftp.mozilla.org/pub/mozilla.org/firefox/releases/
Needless to say with a little ingenuity you can find that they have the same for their other projects like Thunderbird.
http://ftp.mozilla.org/pub/mozilla.org/thunderbird/releases/