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.
Advanced Eyedropper, Color Picker, Gradient Generator and other colorful goodies...
As of Firefox 33, Venkman *no longer works*, as the APIs which it needs have been removed.
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/Firebug extension that allows web developers and designers to easily overlay a web composition over top of the developed HTML.
A quick restartless magnifier to debug styles and web pages.