FireQuery is a collection of Firebug enhancements for jQuery. Requires Firebug 1.3 or greater.
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).
Drupal for Firebug extends the Firebug module to provide Drupal specific debugging and status messages. It allows users to see the processing of the node, form, and user objects and works with the Devel module to display SQL queries and execute PHP from the Firebug window. It incorporates jQuery 1.26 for added UI improvements.
A set of tools to help you develop web applications by investing Firebug with the knowledge of the JS framework you are working with. Supports ExtJS and Sencha Touch, with beta-level support for Dojo Toolkit, SproutCore, qooxdoo, YUI, Google Closure Library, and jQuery UI. Trial version.
The best security you can get in a web browser! Allow active content to run only from sites you trust, and protect yourself against XSS and Clickjacking attacks.
You are most welcome to suggest any additions to this list. You can use the comments box below to make any suggestions.