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)
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.
Pixel Perfect is a Firefox/Firebug extension that allows web developers and designers to easily overlay a web composition over top of the developed HTML.
Checks for compliance with the contrast levels, brightness and shine in the color combination of foreground and background of textual content based on the requirements of WCAG 1 and WCAG 2.
The extension generates a documentmap or index of any web document structured with headings and/or with sections in HTML 5. It shows the headings structure, the errors in the structure (ie. incorrect levels), and it works as HTML5 Outliner too.