One of the coolest things about opensource software is other people can extend it. There are currently 100 extensions for FireFox. Here are three you should check out: FoxyTunes, MiniT, and Sage.
FoxyTunes lets you control your media play from within FireFox (which is suprisingly handy). It works with many players and you can set up custom ones. I’m using it with iTunes. MiniT adds something I’ve been longing for: the ability to use drag-n-drop to reorder browser tabs. Tabbed browsing is the coolest feature of FireFox. Not being able to reorder them was always a problem for me. Now it’s fixed. Finally, Sage is an RSS/ATOM reader that’s dead simple to use. Of the three, I’ve played with it the least, but I think I’m going to be very happy with it.