Aardvark
- By clicking an element you will able to remove, view source, isolate and see element information.
- http://karmatics.com/aardvark/

Console²
- Firefox extension that replaces the JavaScript Console. Display errors filtered by type (Errors, Warnings, Messages), language (JavaScript, CSS, XML) and context (Chrome, Content). Useful for web and extension developers.
- http://console2.mozdev.org/

View Source Chart
- Draws a color, coded chart of a webpage's source code.
- http://jennifermadden.com/scripts/ViewRenderedSource.html

Colour Contrast Analyser
- Reveals the colour contrast of all elements in the DOM.
- http://juicystudio.com/article/colour-contrast-analyser-firefox-extension.php

HTML Validator
- An HTML and SGML validator, based on the HTML Tidy and OpenSP algorithms originally developed by W3C. Checks pages on the fly.
- http://users.skynet.be/mgueury/mozilla/

Dust-Me Selectors
- Finds and display unused CSS selectors.
- http://www.sitepoint.com/dustmeselectors/

Web Developer Extension
- A toolbar with useful functions for website authors. Includes download, full feature list, screenshots, and discussion forums.
- http://chrispederick.com/work/web-developer/

Firebug
- Tools for web development, allow inspect, edit and monitor CSS, HTML and JavaScript live in any web page. Include documentation, FAQs, videos and discussion groups.
- http://www.getfirebug.com/

JS Console Extras
- Adds a JS Console context-menu item which presents a "Toolbars" submenu for hiding or showing the eval / mode toolbars.
- https://addons.mozilla.org/firefox/37/

Tuvbar
- Toolbar extension for helping penetration-testers test the security of websites, networks and webpages.
- http://tuvbar.mozdev.org

LinkChecker
- Check the validity of links in a html document.
- http://www.kevinfreitas.net/extensions/linkchecker/

Michael's Firefox Extensions
- Extensions for HTML, CSS and RSS validation.
- http://www.nu22.com/firefox/

ColorZilla
- A collection of tools, including a Color Picker similar to the one found in Photoshop, an Eyedropper that helps to get a color reading from anywhere in the browser window and a Page Zoomer that helps set the page zoom to anything between 20 and 1000 perce
- http://www.iosart.com/firefox/colorzilla/

cuneAform HTML/XHTML Editor
- cuneAform has been designed to create a HTML Editor that anyone can use.
- http://cuneaform.mozdev.org/home/index.htm

Firefox Extensions for Web Developers
- A list of extensions built primarily for web designers and programmers.
- http://loadaveragezero.com/drx/extensions