One of the best reasons to use Firefox is because there are so many damned good extensions you can add on after the download to enhance your experience. These extensions can provde anything from a few more context menu entries to a complete redo of a fundamental aspect of the browser’s interface. You can do damned near anything with a combination of XUL, JavaScript, and Java, and the extensions show it.
The extension at the top of my list right now is Tabbar Extensions. Adding Tabbar Extensions to FF is nearly as big of a jump as moving from MSIE to FF and discovering tabs for the first time. It groups tabs into natural arrangements based on which tabs were opened from which links, and it not only keeps them together on the bar but color-codes them and allows you to bookmark all of them in a group, to be restored at will. In fact, you can save and restore whole tab sessions. It expands and scrolls the tab bar, too, so each tab keeps a constant size while allowing you to manage ungodly amounts of them at once. (I have had three rows of eight tabs open at once. I’m an addict, I tell you!) You can enable and disable JavaScript, plugins, images, reloading, and frames on a tab-by-tab basis. It also gives you a sorted list of tabs in a simple drop-down menu on your main browser taskbar.
Pure tab addict heaven. 
Interestingly, the same guy has a Tabkiller extension. It removes tabs from FF entirely, something I consider bizarre and unholy but also a cool use of FF’s extensibility.
The final extension I’ll outline in this post is mnenhy, an extension that provides interesting text-manipulation widgets for the context menu. You can rot13 text, uuencode/uudecode it, base64 en- and de-code, even convert it to leet and Kenny. Oh, and you can reverse it, too, and evaluate mathematical expressions. The effect, when applied to normal text, can be surreal.
If you want to dip your toes into the wide world of making Firefox a bloated, overfeatured mess, check out Mozdev.org.
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