http://sfx-images.mozilla.org/affiliates/Buttons/180x60/blank.gif∞

Greasemonkey∞ is a FireFoxExtension that lets you add extra JavaScript to any and every web page you visit

Actually GreaseMonkey doesn't do anything (apart from add a monkey button and three menu commands until you install GreaseMonkeyUserScripts !)

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REFERRERS
FireFoxExtensions
GreaseMonkey
GreaseMonkeyUserScripts