Do you like being able to virtually grab a page and scroll up or down it in your browser? Now, some other programs have this ability (such as most PDF readers out there) however the web browser does not, until now. The Grab and Drag add-on for Firefox will enable grab and drag scrolling, flick gestures, and momentum scrolling in Firefox.
This comes in really handy for tablet PCs, where your pen is taking the place of your mouse. You can though use it with your mouse too, as it comes ready for 1, 2, or 3 button mouse support.