Firefox 3 has been out for a while now and if you're sitting on the fence about the upgrade, I will personally recommend it, if only for Facebook.
My friends are bloody Facebook application mad, and opening more than one profile at a time was causing Firefox 2 to crawl. That's where Firefox 3 comes in. With the new version they've put major effort into rendering performance, and the difference is as clear as night and day.
Give Firefox 3 a spin on any AJAX enabled site and you'll immediately feel the difference. The only question I have is this: Why did we have to wait this long for a decent browser?
No use in whining however, it's out and available, so download and enjoy!