Protect your privacy. See who's tracking your web browsing and block them with Ghostery.
The Web Developer extension adds various web developer tools to the 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...
View HTTP headers of a page and while browsing.
Draw a ruler across any webpage to check the width, height, or alignment of page elements in pixels.
Wappalyzer is a browser extension that identifies software on websites.
Allows you to quickly switch to a different language (User Interface, Spell Checker Dictionary and Website content) in your Mozilla application.
Pencil is a Firefox add-on to do GUI prototyping and simple sketching
For Sysadmins / Network Engineers / and why not everybody who needs a quick password generator.. simple, yet efficient.