Haven't really noticed a lot of new features with FF3 yet, its standard design is a bit nicer imo and it loads a little faster, there is one "improvement" though that almost drove me insane. Instead of the old location bar - you know where you type in an url - they have invented something they call "The Awesome Bar" which in my eyes isn't very awesome at all! Instead of giving you URLs you have visited that starts with 'E' when you type in an 'E' in the bar, it lists all sorts of pages from your history that has an E in the content or somewhere in it's title. Which is really frustrating when you have gotten used to how a location bar works after using a computer for well over a decade.
Luckily I found this plugin that gives me back a non-awesome bar, which works the way I like it to.
(
https://addons.mozilla.org/en-US/firefox/addon/6227) Otherwise I would have switched to Opera or something..