One of my must-have extension for Firefox is Web Developer from Chris Pederick. It has lots of options which is quite useful when you are building a web application. You can simulate a screen view in other resolution, view almost all kind of information during the development of the web pages, such as table information, table borders, images, CSS, Forms, and many more. I recommend this extension if you are a web developers or someone who love to make web application. Click here if you want to install Web Developer for Mozilla Firefox/Flock or here if you want to install it on Mozilla/Seamonkey.
It's not the only extension i recommend, since there are a lot of them, but in my personal opinion, this is my #1 extension.