Firefox 3 has majorly improved, most notably by being quicker and much lighter in memory usage. They've also updated the UI to better match the operating system it's running in. Big functionality changes are evident in bookmarking, the location bar, downloads, and addons. Basically they've made major improvements across the board. Also, a lot of reviews have suggested it's gone from unusable in Mac OSX to a browser that now is much better than Safari. In less than two hours you can see for yourself!
Still not convinced? Check out some of Ars Technica's reviews:
Memory Usage
User Interface
And one for Mac users about exactly why this blows away FF2.
Update: The website went down, and they posted a message saying Download Day will begin as soon as the site is back up and run for 24 hours from then.
Posted by: Anonymous on Jun 17th, 2008 | 11:25am
Also, from that Mac article:
"If, like me, you often have 10 or more tabs open at once, then quitting Firefox 2 in a hurry is a pain."
Why does he have to quit Firefox in a hurry so often?