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

In the same way EditCSS lets you modify the CSS of any web page GreaseMonkey lets you add(/modify?) the JavaScript(s) (GreaseMonkeyUserScripts) on each and every (or your choice) web page you visit !

Ensure you have GreaseMonkey installed then start downloading GreaseMonkeyUserScripts∞ today

Recommended

Expand Text Area: allows you to resize textareas.
Zoom Image: Displays a small popup menu over images (in the top left corner) that allows zooming.

If you know JavaScript then why not write your own GreaseMonkeyUserScripts !

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REFERRERS
FireFoxExtensions
GreaseMonkey
GreaseMonkeyUserScripts