clipped from: www.listible.com 
"Greasemonkey is a Firefox extension which lets you to add bits of DHTML ("user scripts") to any web page to change its behavior. In much the same way that user CSS lets you take control of a web page's style, user scripts let you easily control any aspect of a web page's design or interaction."
The development of Adblock pretty much stopped in 2004. When more and more people demanded improvements Adblock Plus was created. Adblock Plus is a completely rewritten version of the original Adblock.
Loads of Tab improvements. Changing the behavior of tab opening, focus, closing, mouse events on tab and much more. One of the best tab extensions around!
This extension automatically fills in login forms from a list of username and passwords collected from bugmenot.com. With this, you can bypass registrations on websites that you would not like to be registered with will still being able to access their content.
This extension allows you to automatically keep up to date Adblock or Adblock+ with G's latest filterset every 4-7 days. It also provides you with the ability to define your own filters that you want to persist between updates.
"StumbleUpon uses ratings to form collaborative opinions on website quality. When you stumble, you will only see pages which friends and like–minded stumblers have liked."
ScrapBook is a Firefox extension, which helps you to save Web pages and easily manage collections. Key features are lightness, speed, accuracy and multi-language support. Major features are:
* Save Web page
* Save snippet of Web page
* Save Web site (In-depth Capture)
* Organize the collection in the same way as Bookmarks tree
* Highlighter, Eraser and various page editing tools
* Full text search and quick filtering search
* Text edit feature resembling Opera's Notes
It enhances Google search results by adding extra information (like links to Yahoo, Ask Jeeves, MSN etc) and removing unwanted information (like ads and spam).
Manage a lot of things through the Firefox sidebar, create new toolbars, etc.
Adds zoom functionality for images. Easily zoom in, zoom out, fit image to screen or set custom zoom on individual images within a web page.
This extension allows you to execute common commands using mouse gestures, rocker navigation, scroll wheel navigation and page scrolling.