Provides searchable reference information and code examples for HTML and CSS
FireXPath is a Firebug extension that adds a development tool to edit, inspect and generate XPath expressions.
Track Changes in Firebug
Add TortoiseSVN commands to link context menus to allow easily browsing Subversion repositories. Must have TortoiseSVN already installed.
Firecookie is an extension for Firebug that makes possible to view and manage cookies in your browser
Detects what javascript libraries are being used on the current page and displays the result as icons with detailed tooltips in the statusbar.