1. Web Developer Toolbar

The must have tool for any web developer.
Download Web Developer Toolbar

2. Firebug

Another must have extension for any developer. View, edit, debug, and monitor CSS, HTML, and JavaScript live in any web page.
Download Firebug

3. View Source Chart

Draws a color coded chart of a web page’s source code and includes automatic indentation to make source code easier to read.
Download Source Chart

4. HTML Validator

Adds HTML validation to the view page source of the browser.
Download HTML Validator

5. SEO Quake

Quick view of site parameters in the search engine results pages
Download SEO Quake

6. ColorZilla

This extension adds a small eyedropper tool to the bottom left of the status bar which when clicked upon allows you to hover over any part of a web page to view the colour that is being used.
Download ColorZilla

7. Colour Contrast Analyser

A nice accessibility extension to check whether there is sufficient contrast between foreground and background colours of a web page.
Download Colour Contrast Analyser

8. Foxmarks

The Foxmarks Bookmark Synchronizer automatically synchronizes your bookmarks between two or more computers running Firefox.
Download Foxmarks

9. Adblock

Not so much a development tool but useful nonetheless. This extension allows you to block adverts within web pages easily and effectively.
Download Adblock

10. Dust Me Selectors

Dust-Me Selectors is a Firefox extension that finds unused CSS selectors.
Download Dust Me Selectors