Anything you might want to use to be a highly productive, happy web worker.
Restyle the web with Stylish, a user styles manager. Stylish lets you easily install themes and skins for Google, Facebook, YouTube, Orkut, and many, many other sites. You can even customize Firefox and other programs themselves.
The Web Developer extension adds various web developer tools to the browser.
YSlow analyzes web pages and suggests ways to improve their performance based on a set of rules for high performance web pages.
http://yslow.org is the official open source website.
The extension previously known as GooglePreview. Inserts preview images (thumbnails) and popularity ranks of web sites into the Google, Yahoo, Bing and DuckDuckGo search results pages.
CodeBurner is a Firefox add-on that integrates with Firebug, to extend it with reference material for HTML and CSS.
Automatically find user scripts on Userscripts.org (requires Greasemonkey or Scriptish)
Shareaholic is the easiest way to share interesting links using Facebook, Twitter, Google Mail, Reader, Bookmarks, Evernote, Bitly, StumbleUpon, and more... from one simple ALL-IN-ONE add-on. This is the ultimate tool for the link sharing addict!
Firecookie is an extension for Firebug that makes possible to view and manage cookies in your browser
Firecookie is part of Firebug since Firebug 1.10
Add useful extra features to Gmail, like hierarchical labels, an unread message icon on your browser tab, file attachment icons, and more.
Support web developers in testing web resources for accessibility features.
SenSEO is an extension checking the most important on-page-SEO-criteria and calculating a grade of how good your site fulfills these criteria.
Note from Mozilla: This add-on has been discontinued. Try PDF Viewer instead.
PDF plugin for Firefox 3 on Mac OS X. Displays PDF documents in the browser.
Collections are groups of related add-ons that anyone can create and share.Explore Collections