They’re actually trying to set a Guinness (Mmmm, Guinness) world record for most software downloads in a day.
You can get more info on that, and pledge to help set the record, at: spreadfirefox.com/worldrecord/
Deb Richardson has a writeup of all the new features and improvements in FF3 here.
Personal favs include the faster rendering (up to 4x faster than FF2, 9X faster than IE7), site identification bar, and the nice little touches to your browsing history and password manager.
MailChimp is making some improvements (coming soon in MailChimp 3.1) that will work with some of the new features in FF3. More on that later.