Firefox 3 has a list of improvements and new features. Mozilla sums them up as such:
Keeping you and your personal information safe is our top priority. Firefox 3 includes phishing and malware protection, plus our new instant site ID info.
With features like built-in spell checking, session restore and full zoom, Firefox 3 makes it possible to work faster and more efficiently on the Web.
Your taste and needs set you apart from the rest. With Firefox 3 you can choose from over 5,000 add-ons that help you customize your browsing experience.
The awesome bar improved over the course of the various test releases, from a rather unclear mess to something where entries' details are highlighted according to importance. The awesome bar still has a nasty habit of favouring the wrong links over the right ones, but it has been getting better with each test release. For instance, with older releases, typing in "web" would give as a first hit some online store with "webobjects" in its url that I visited once, instead of bringing up "webmail.student.vu.nl", my university's webmail which I visit several times during the day.
Performance-wise, Firefox 3 also appears to be a major stride forward. Firefox 2.0 was a memory hog, and slow to load upon first boot. Firefox 3 feels more like Safari in that it loads almost instantly, and loads pages faster too.