Alex Sirota's FoxyTunes embeds controls for a heap of music players directly into Firefox's user interface.
One area in which Mozilla/Firefox really has it over other browsers is the ability to actually extend the browser. Previously, in pondering Mozilla extensions for client side control of page display, I mused that Mozilla is way easier to extend than IE, but not trivial. Maybe the threshold is a lot lower than I thought. Even if they're not hard to write, they're a darn site easier to install.
Currently, I have my browser kitted out with the TabBrowser, Live HTTP Headers, and WebDevelopers extensions and I'm going to give FoxyTunes a try. And I'm probably an extension lightweight.
It's going to be interesting if Mozilla/Firefox ushers in a new era of extensible browsers. For a while there, toolbars and side bar development was really hot. Still could be in IE land for all I know. But think about potential synergies between in-page applications (e.g. Flickr, GMail) and in-gui extensions. While the browser might never become the MS killing platform of Netscape high times, there's definitely going to be some serious drag in moving to those rich media desktopish apps.