Add-ons and toolbars to help test a page for web accessibility.
Blocks annoying video ads on YouTube, Facebook ads, banners and much more.
Adblock Plus blocks all annoying ads, and supports websites by not blocking unobtrusive ads by default (configurable).
The Web Developer extension adds various web developer tools to the browser.
Advanced Eyedropper, Color Picker, Gradient Generator and other colorful goodies...
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 Firefox 23+, you need 0.958 (under review)
Support web developers in testing web resources for accessibility features.
The toolbar enables developers to examine WAI-ARIA live regions roles and properties, examine data tables, and determine if the colour contrast is sufficient.
Web Accessibility Test.
You can have an idea, with a single click, of how accesible web sites are.