There are hundreds of add-ons and extensions which let firefox do extra things beyond the usual browsing stuff. The problem is knowing which are worth having and which are just clutter that might slow things down.
A lot of the add on bits duplicate each other, so if you have one which does what you need, you don't need a second one which does the same unless it does it better.
Some work with earlier versions of Firefox, but not with the Firefox 3, or vice-versa so here would be a good thread to swap ones any of us have found useful, or to warn of extras that turn out to be not as good, or just clutter.

My first suggestion for Microsoft PC users is Portable Firefox part of a suite of applications from http://portableapps.com/.
The portable applications are designed to load onto a USB stick so you can take them with you and work on other computers without having to worry whether the computer already has compatible programmes installed. It doesn't install ANYTHING into the PC Registry, everything it needs is installed into the one folder which is free-standing. This also means that you can put the portable installation on ANY drive on your PC and run it. If you want to uninstall, all you do is delete the folder. If your PC needs a complete reinstallation of Windows, as long as the folder is saved on a second hard drive, it will still be there waiting to be used.
With Portable Firefox on a USB stick, you have all your bookmarks with you if you are mobile, very handy for working across two computers in the same house, or at work, as long as you do not lose the stick.

Other tips to follow as separate postings to avoid confusion.
Quack!
GtD.