Here is a list of my favourite Firefox plugins.

Web Developer
The Web Developer extension adds a menu and a toolbar with various web developer tools. If you do any sort of web development using HTML or CSS you absolutely need this plugin. There is no debate about it.

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...

Xmarks Bookmark and Password Sync
Xmarks is a bookmark and password syncronisation add-on. You can install it on all your computers to keep your bookmarks and (optionally) passwords backed up and synchronized. Xmarks also allows you to see what sites other people are bookmarking.

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

DownloadThemAll!
The first and only download manager/accelerator built inside Firefox!

Aviary Screen Capture
This addon for quick screen capture is a must-have for bloggers and designers alike! It will perform a screen capture and then open the image INSTANTLY in a basic image editor where you can do markup (draw arrows and rectangles), edit (crop, rotate and resize) and get the exact pixel colors of the image. A perfect companion tool for designers and bloggers!

IE Tab
Allows you to display a tab using the IE rendering engine. A great tool for web developers, since you can easily see how your web page displayed in IE with just one click and then switch back to Firefox.