Music Player Minion is a client for the Music Player Daemon network music player. The purpose of MPD and it’s clients is to allow music playback on one PC (such as a home media server) to be controlled from another over the network.
Here is a little more about the add-on from the developers:
This extension provides a full client, a sidebar version and a mini-player. You’ll need to enter the mpd host and port information in Settings if you are controlling a remote server. You can control multiple MPD instances by dconfiguring multiple servers.
Want to learn more? You can pick up the Music Player Minion from the Firefox Add-ons website.