After installing FireFox (FF) the first thing I do (normally every developer will do) is search and install the addons.
Here are some handy addons a web developer must install.
1. Firebug

Firebug is considered to be the ultimate developer’s tool for Firefox. You are given a ton of web development tools to use from within your browser. You can explore, edit, debug, and monitor CSS, HTML, and JavaScript in real-time on any web page.
2. YSlow

YSlow analyzes web pages and why they’re slow based on Yahoo!’s rules for high performance web sites.
3. Coral IE tab
If u develop website in FF and also care for IE users then u will love this one. It enables you to use the embedded IE engine within tabs of Mozilla Firefox.

The Web Developer extension adds a menu and a toolbar with various web developer tools.
5. Measure It

Draw out a ruler to get the pixel width and height of any elements on a webpage.
6. ColorZilla

Advanced Eyedropper, ColorPicker, Page Zoomer and other colorful goodies.
7. X – Ray
See the tags on a page without viewing the sourcecode.
8. Font Finder

FontFinder is created for designers, developers and typographers. It allows a user to analyze the font information of any element on a page, copy any piece(s) of that information to the clipboard, and perform inline replacements to test new layouts.
9. Professor X
The Professor X Extension let’s you see inside a page’s head without viewing the sourcecode.
10. Aardvark

Cool utility for cleaning up a page (for printing or better viewing) as well as for web development. Displays a red rectangle on the element under the mouse — hit “w” to go wider (get the containing element), then hit other keystrokes to remove, etc.
Which Firefox Addons do you use?
Just leave a reply and tell us about your favourite FF add-ons.