When I first heard about Mouse Gestures, I was skeptical about installing the add-on. Now, I can firmly say that using Mouse Gestures optimizes your browsing by a factor of at least 2x.
The idea behind Mouse Gestures is that when you hold down the mouse button, move in a direction, then let go of the mouse button, that particular motion performs a certain action. The action could be move forward a page, back up a page, or reload the page.
I guess it takes a while to get used to, but once you feel comfortable, it actually gives the user a quick way to move around web sites. This would be the mouse equivalent of the keyboard-jockeys who use only the keyboard in Firefox to get their work done because they think it's faster (That's another religious question, which will be discussed at a later time).
Mouse Gestures receives a 5/5.