The Web Developer extension adds various web developer tools to the browser.
View HTTP headers of a page and while browsing.
Cookies manager to view, edit and create new cookies. It also shows extra information about cookies, allows edit multiple cookies at once and backup/restore them.
CodeBurner is a Firefox add-on that integrates with Firebug, to extend it with reference material for HTML and CSS.
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)
DOM Inspector is a tool that can be used to inspect and edit the live DOM of any web document or XUL application. The DOM can be navigated using a two-paned window displaying a variety of different views on the document and all nodes within.
User Agent Updater Enables you to update User Agent immediately through context-Menu
CSS Reloader is an extension that allows you to reload all the CSS of any site without you have to reload the page itself.
Syntax highlighting for the Firebug command line using Ace.
Fuzzy auto completion