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.
The User Agent Switcher extension adds a menu and a toolbar button to switch the user agent of a browser.
Advanced Eyedropper, Color Picker, Gradient Generator and other colorful goodies...
IE Tab - an extension from Taiwan, features: Embedding Internet Explorer in tabs of Mozilla/Firefox...
Draw a ruler across any webpage to check the width, height, or alignment of page elements in pixels.
Finds HTML elements matching chosen CSS selector(s) or XPath expression
DOM Inspector is a tool that can be used to inspect and edit the live DOM of any web document or XUL application. The DOM can be navigated using a two-paned window displaying a variety of different views on the document and all nodes within.