A collection of useful web development tools. For an almost complete list of Firebug addons see: https://addons.mozilla.org/de/firefox/collection/firebug_addons
The Web Developer extension adds various web developer tools to the browser.
Use tamperdata to view and modify HTTP/HTTPS headers and post parameters...
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.
FontFinder is created for designers, developers and typographers. It allows a user to analyze the font information of any element on a page, copy any piece(s) of that information to the clipboard, and perform inline replacements to test new layouts.
FirePHP enables you to log to your Firebug Console using a simple PHP method call.
Not using Firebug 1.9+ on Firefox 8+? See http://firephp.org/
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
The extension generates a documentmap or index of any web document structured with headings and/or with sections in HTML 5. It shows the headings structure, the errors in the structure (ie. incorrect levels), and it works as HTML5 Outliner too.