Update: We've also run Kraken 1.1 and the v8 Benchmark Suite version 6 on the same browsers - these numbers come a bit closer to realizing the 30 percent improvement over version 8, while even more conclusively demonstrating how far Firefox has come since version 3.6.
Firefox 9 also brings two-finger navigation to users running OS X Lion - swiping left on a multitouch device will go back, and swiping right will go forward. Safari and Chrome have already implemented this feature, and while the functionality is the same in Firefox, those browsers include visual cues (arrows in Chrome, a visual sliding of the page in Safari) to let you know that you're doing it, visual feedback that I find helpful. There's still no support for fullscreen mode, though plugins exist to give this functionality to those who want it.
The rest of the improvements can be found in the usual laundry list of small security and bug fixes (including one I had run into where attachments wouldn't download properly when using the Outlook Web App - useful information for shops running an Exchange server). You can get the new browser by using Firefox's built-in updater or by visiting getfirefox.com.
Note: At the moment, Mozilla says that it's not pushing Firefox 9 out to OS X and Linux users automatically because of "a possible issue affecting a small number of users." We'll keep an eye on this and update the post as events progress.