There is a good list of 20 must-have Firefox extensions published at computer world web site. These plug-ins give you souped-up functionality, better look and feel, and streamlined development tasks.

Currently I am using following two add-ons
1) Google Toolbar
2) Stumble! Toolbar

You can find some other Firefox add-on information here :)

A freshly installed copy of Firefox is a great software package, but what makes this open-source browser so special is the ability to customize it via extensions and themes to really make it yours. The problem is, there are so many available add-ins, it's tough to know what's worth installing and what's just going to junk up your system.
* Tools for taming the Web
* Visual improvements
* Matters of convenience
* Information gatherers
* Web developer essentials

=> 20 must-have Firefox extensions