DOM Inspector is a tool that can be used to inspect and edit the live DOM of any web document or XUL application. The DOM can be navigated using a two-paned window displaying a variety of different views on the document and all nodes within.
HTML Validator is a Mozilla extension that adds HTML validation inside Firefox and Mozilla. The number of errors of a HTML page is seen on the form of an icon
For Online HTML 5 validation, you need 0.962
(Formerly Local Install) The primary goal of this extension is to provide the tools needed to install and manage extensions and themes locally and tons of other features.
Note: With Toolkit 6.0 Auto Archiving is fully working in Firefox 2.0 and temporarily disabled for Firefox 3.0, fix coming soon.
Toggle Web Developer Toolbar has been specially designed for web developers to provide easy to use methods to toggle Web Developer toolbar so that they can preserve their browser view area when they do not use Web Developer toolbar.