The Web Developer extension adds various web developer tools to the browser.
The User Agent Switcher extension adds a menu and a toolbar button to switch the user agent of a browser.
Use tamperdata to view and modify HTTP/HTTPS headers and post parameters...
Advanced Eyedropper, Color Picker, Gradient Generator and other colorful goodies...
View HTTP headers of a page and while browsing.
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).
Draw a ruler across any webpage to check the width, height, or alignment of page elements in pixels.
Seoquake is a Firefox SEO extension aimed primarily at helping web masters who deal with search engine optimization (SEO), social media optimization (SMO) and internet promotion. Seoquake allows to investigate many important SEO parameters.
This plugin is my attempt to bring the wonderful world of inspections, tips, hints, fixes and refactoring to Selenese! The Selenium Expert goes through your selenium test cases suggesting improvements, giving tips and even lets you apply them.