Here is a list of Firefox Addons that I use

Adblock Plus:
Adblock is an essential part of webbrowsing in todays web. It's probably the most controversial addon I use because "the web is not free". It's a long story, google it, come to your own decision.
Firebug:
This addon isn't for everyone, but if you've ever needed to debug a webpage then you might have some idea of how valuable this can be. Firebug allows you to scrutinize a webpage down to the tiniest detail, it's extremely powerful and and absolute must if you do any web programming.
Download Them All!:
This is an essential tool if you download files at all. Download Them All! supports file resume, segmented downloading, and has some very powerful filters. A must if you've ever had a large file cancel on you.
Foxmarks:
If you have more than one computer or use more than one computer you know how much you want to keep your bookmarks in sync. This program does exactly that and does it well. You can also access your bookmarks online for those times when you can't install the addon.
PDF Download
PDFs can be very annoying to manage, this addon will help you by prompting you whenever you encounter a PDF.