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From the Authors:
Mouse Gestures are a fast way to execute commands without using the keyboard, menus or toolbars. Instead, the user holds down a mouse button (usually the right one), moves the mouse in a certain way to form a gesture, then releases the mouse button.
In web browsers like Mozilla Firefox or the Mozilla Suite, gestures are used to go back or forward a page, switch between tabs, open multiple links at once, control text or image size, and numerous other functions.
However, gestures are in no way limited to browsers: the Mouse Gestures extension also supports Mozilla Thunderbird, Mail and News (Mozilla's built-in email client) and Chatzilla.
This should be an effective add on for the Firefox web browser for people who mainly use the mouse - for example people using a head pointer or trackball.