The Web Developer extension adds various web developer tools to the browser.
Draw a ruler across any webpage to check the width, height, or alignment of page elements in pixels.
YSlow analyzes web pages and suggests ways to improve their performance based on a set of rules for high performance web pages.
http://yslow.org is the official open source website.
Allows you to quickly switch to a different language (User Interface, Spell Checker Dictionary and Website content) in your Mozilla application.
Control what gets sent as the HTTP Referer on a per-site basis.
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.
NoRedirect lets the user take control of HTTP redirects. It can be used to interdict an ISP's DNS search redirection hijacks, preview/screen "shortened" URLs (e.g., TinyURL), stop the annoying redirection of "smart" error pages, etc.
Firecookie is an extension for Firebug that makes possible to view and manage cookies in your browser
Firecookie is part of Firebug since Firebug 1.10