Firefox useful addons

Firefox is most powerful browser i ever seen. I said powerful because of feasibility of implementing Add-ons/extensions (Add-ons are small pieces of software that can add new features or tiny tweaks to your Firefox.). I use Firefox because of these Add-ons/extensions.
They helped me lot in my daily office work. It has enormous features with the help of Add-ons/extensions.
Here I am listing the powerful/most useful Add-ons/extensions list.

1. Greasemonkey

Allows you to customize the way a webpage displays using small bits of JavaScript.
Hundreds of scripts, for a wide variety of popular sites, are already available at http://userscripts.org.

You can write your own scripts, too. Mark Pilgrim's definitive Greasemonkey guide, diveintogreasemonkey.org will show you how.

2. Web Developer -

Adds a menu and a toolbar with various web developer tools.

3. Download Statusbar:

see all preview images

View and manage downloads from a tidy statusbar - without the download window getting in the way of your web browsing.

Despite its compact size, Download Statusbar packs in more useful features than the standard download window. The fully customizable interface auto-hides when not in use, allowing full control without interruption.