The Web Developer extension adds various web developer tools to the browser.
FirePath is a Firebug extension that adds a development tool to edit, inspect and generate XPath 1.0 expressions, CSS 3 selectors and JQuery selectors (Sizzle selector engine).
Debug Flash/SWF files on the web. Edit properties and inspect elements on the fly. View log output inside the extension. Run AS3 code against the active SWF. Access assets with the Explorer. View AMF calls and Shared Objects and much much more...
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/
CodeBurner is a Firefox add-on that integrates with Firebug, to extend it with reference material for HTML and CSS.
Finds HTML elements matching chosen CSS selector(s) or XPath expression
Edit your css with FireBug and let cssUpdater update the corresponding property and value in your css source file. Pixel push with ease! (Please visit the homepage for more information.)