I installed Firefox 3.1b2 on my MacBook this evening to a pleasant surprise -- it now supports a variety of multi-touch trackpad gestures as seen here. I remapped two of them slightly, however -- three finger swipe to scroll to the top/bottom of the page wasn't useful for me, and I find the twist gesture to be kind of awkward. So, I remapped that to changing tabs.
It's a very nice feature and I'm glad to see they've added it.