Open Dom Inspector at this node from Inspect on context menu. Dom Inspector must already be installed. Handy for checking both page for web developers and chrome for extension...
Create and manage add-ons made from Greasemonkey and Scriptish user scripts
A Regular Expression testing tool
Allow RegExp developer to test the regular expression on the live HTML content. Useful for those who work on crawling website data.
Syntax highlighting for the Firebug command line using Ace.
Fuzzy auto completion
Feature rich XPath generator, editor, inspector and simple extraction tool...
IDE for FireFox plug-in developers and business analysts who create User Interface mock-ups. A WYSIWYG XUL editor
Adds toolbar button and context menus for invoking the DOM Inspector (DOMi) for either chrome or content elements.
Holmes is stand-alone diagnostic CSS stylesheet that can highlight potentially invalid, inaccassible or erroneous HTML(5) markup by adding one class.
A simple add-on for extension testers, especially AMO editors and developers who plan to submit their add-ons to AMO.
FireLESS allows Firebug to display the original LESS file name and line number of LESS-generated CSS styles.
It currently requires a patched version of LESS available at https://github.com/simonedeponti/less.js.