The Web Developer extension adds various web developer tools to the browser.
Advanced Eyedropper, Color Picker, Gradient Generator and other colorful goodies...
CodeBurner is a Firefox add-on that integrates with Firebug, to extend it with reference material for HTML and CSS.
As of Firefox 33, Venkman *no longer works*, as the APIs which it needs have been removed.
Pixel Perfect is a Firefox/Firebug extension that allows web developers and designers to easily overlay a web composition over top of the developed HTML.
A suite of tools for developers.