14 Add-ons in this Collection
Firebug by Joe Hewitt, Jan Odvarko, others
The User Agent Switcher extension adds a menu and a toolbar button to switch the user agent of a browser.
The Web Developer extension adds various web developer tools to the browser.
As of Firefox 33, Venkman *no longer works*, as the APIs which it needs have been removed.
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 Online HTML 5 validation, you need 0.962
Scriptish by Erik Vold, Nils Maier, others
The greatest user script engine on the Internet (a fork of Greasemonkey).
Pixel Perfect by Mike Buckley, Lorne Markham, others
Pixel Perfect is a Firefox extension that allows web developers and designers to easily overlay a web composition with semi-transparent layers (images). These layers can be consequently used for per pixel comparison between the page and a layer.
A quick restartless magnifier to debug styles and web pages.