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.
Use tamperdata to view and modify HTTP/HTTPS headers and post parameters...
Advanced Eyedropper, Color Picker, Gradient Generator and other colorful goodies...
Lightbeam is a Firefox add-on that enables you to see the first and third party sites you interact with on the Web. Using interactive visualizations, Lightbeam shows you the relationships between these third parties and the sites you visit.
Notice: some important features (XML indentation and load old saved requests) are missing in version 2.0.0, please update to version 2.0.1.
Please follow the link ("View other versions") below To install version 2.0.1.
View HTTP headers of a page and while browsing.
Manages the Remote XUL whitelist to allow websites and local files to load XUL content.
Adds an about:addons-memory page that details add-on memory usage
As of Firefox 33, Venkman *no longer works*, as the APIs which it needs have been removed.