From Music Player Daemon Community Wiki

Music Player Minion is a XUL client for MPD. It is packaged as a firefox extension, providing both a sidebar version and a full player. Even though it is built for a browser, no web server is required for this client. Please go to [http://code.google.com/p/musicpm/] for more information.

UPDATE: This extension is now available for install from Mozilla Addons at [https://addons.mozilla.org/en-US/firefox/addon/6324]