Mozilla Firefox is becoming more and more powerful tool in these days. Using right add-ons it will start to become a tool, You can`t live without as web-developer! Each of these plugins can make certain things easier and that`s 
especially important for web-developers
1. ColorZilla
ColorZilla offers You an advanced eyedropper, colorPicker, page Zoomer and other colorful goodies.
2.Delicious Bookmarks
Delicious bookmarks is official Firefox addon for del.icio.us. It integrates your browser bookmarks and tags, to keep them in order and “easy to find” way.
3. Domain Details
Displays Server Type, Headers, IP Address, Location Flag, and links to Whois Reports. Shows links to check server status and cache when page fails to load.
4. DOM Inspector
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 hierarchy can be navigated using a two-paned window that allows for a variety of different views on the document and all nodes within.
Check my firefox plugins scre
enshot.
5. Firebug
Firebug integrates with Firefox to put a wealth of development tools at your fingertips while you browse. You can edit, debug, and monitor CSS, HTML, and JavaScript live in any web page…
6. Firephp
FirePHP enables you to log to your Firebug Console using a simple PHP method call.
7. IE NetRenderer
Render the current page with IE NetRenderer
8. JavaScript Debugger
A powerful JavaScript debugging environment for Mozilla based browsers.
9. JavaScript Options
Provides advanced JavaScript options for Firefox.
10. MeasureIt
Little Firefox extension that enables you to draw a ruler across any webpage to check the width or height of a element.
11. ScreenGrab!
Screengrab saves entire web pages as images. This tool will allow You to take screenshots of what is currently on your screen – you can even choose the portion of the screen you want to “grab”!
12. Speed dial
Direct access to your most visited websites
13.StumbleUpon
StumbleUpon helps you to find new sites very easy.If you are a web-developer, StumbleUpon can drive a huge amount of visitors to your website.
14. View Cookies
It adds a tab to the Page Info dialog box, which shows the cookies belonging to the current page.
15. View Source Chart
HTML Source Code GUI
16. Web developer
Web Developer`s tool bar probably will become one of the best plugins You`ve ever seen. It comes with his own tool bar that gives many useful tools for analyzing, validating and optimizing web-pages. The great thing about this plugin is that it’s on a few different browsers, so if you switch between them you will be able to use the same plugin on all.