The Web Developer extension adds various web developer tools to the browser.
Advanced Eyedropper, Color Picker, Gradient Generator and other colorful goodies...
HTML Validator is a Mozilla extension that adds HTML validation inside Firefox and Mozilla. The number of errors of a HTML page is seen on the form of an icon
For Firefox 23+, you need 0.958 (under review)
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.
Allows you to hide almost anything via context menu 'Remove Object'.
Fangs renders a text version of a web page similar to how a screen reader would read it. The ambition is to help developers understand how an assistive device would present a website and thereby increase chances of finding accessibility issues early.
This is a cut down version of IE View by Paul Roub, which is 47.4 KB. All the same UI and features are there. It has a right click menu item to open a page in IE and a list of sites to always open in IE. It can close tabs automatically and send...