After you have installed Firefox, there are a few add-ons that I think everyone should use, since they are just that good.

Adblock Plus
This is a great tool to hide advertisements. Nobody wants to look at that crap, and it's a waste of bandwidth.

Adblock Filterset.G Updater
This automatically updates the filter list for Adblock Plus, and it has a couple other small enhancement, such as killing Intellitxt ads.

Flashblock
This stops all flast content from running, and shows you a placeholder instead that you can click to load the flash content. It's great for killing the silly audio players on myspace.

RefControl
This tool allows you to send different HTTP referrer information when requesting pages from certain sites. For example, it allows you to get around image hotlink blocking on websites such as imageshack.

IE Tab
This gives you the option to right click a link and open it in a new tab that is rendered in Internet Explorer. It is really just for web developers that want to check browser compatibility. Normal end users generally have no reason to run IE, since any web site that does specifically require IE is now considered to be shoddy web programming.

ReloadEvery
This tool allows you to automatically reload a web page every X amount of time. It's great for web developers that have dual monitors. Put your browser in one and set it to auto reload every 5 seconds, then code away in the other window. No more clicking the reload button to see your changes.