Yes, there are at least two addons that do mouse gestures. I don't know which, if any, support FireFox 2.0 yet.
Cool. I'll have to check into it.
I like the small graphic changes & the built in spell checkery thing
Google Toolbar (pretty much) made Firefox have a built in spell checker. I sort of suspect that it is/was more functional and accurate than the one that comes with firefox now as well. The way it did spell checking was awesome. That's one of the things I miss about IE/Firefox... google toolbar was awesome.
What is a mouse gester?
In Opera, you hold your right mouse button down after clicking in some arbitrary place on the page and drag it in a specific direction or series of directions. From that, the browser performs some action which is assigned in the same way the rest of the controls are set up. For example, left is back, right is next, down is new tab, down and then up is duplicate current tab, etc. There was also one that closed the current tab, which got me into some trouble a while back. It's since been disabled, though.