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.

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.