Fire.fm - Winner of the Extend Firefox 3 contest, Fire.fm is your direct access to the extensive music library on Last.fm. Listen to music related to your favorite artist, and discover new artists and music in the process. Listen to your friends' favorite music. Find Last.fm users with musical taste similar to your own. Quickly access your favorite stations by just typing a few letters into the location bar. This is the ultimate add-on for the music junkie.

FoxyTunes - Controls media players without leaving the browser. Control any media player while surfing the Web and more...

GoMuzik - Search your favourite free songs at GoMuzik and playit anywhere.
Create your customized music album from more then million songs. There is no limit on number of songs you can add to playlist. We provide Hindi, English, punjabi, Tamil etc songs at GoMuzik and we continuously update our search engine to search for various songs in different languages.
GoMuzik provides simple user interface to search and create your playlist and we try to find the accurate songs at lightening speed.

Lala Control - While there is no official way to control the Flash-based Lala Music Player, the Lala Control Addon dynamically searches through Firefox's open tabs and, if it finds one with Lala loaded, toggles the playback of the Music Player. In order for the Addon to work: *A tab must be opened to http://www.lala.com/
*There must be a song already playing or songs waiting to play in the queue. You can toggle the player by either clicking the status bar icon or using the Ctrl+Alt+S hotkey.

Music Hutch Toolbar - This toolbar works exclusively with music and videos on The Music Hutch, and with it you can shuffle and search all of the files for playback.

Music Player Minion - 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. 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.

My Opus Radio - India's first internet radio station for international music.

Online Music Player - This music player installs down in your status bar and allows you to play MP3 files either from the web or from local files.  It is also capable of displaying lyrics and managing playlists.

Play Them All - Play Them All scans all of the links on a page for ones that end in .mp3 and creates a M3U playlist from them so that you can easily play them.

Radio Eksen - This add-on will allow you to see what is currently playing on Radio Eksen, a rock radio station in Istanbul, Turkey.  You can click on the display to listen to the station in your media player, check the artist out on Wikipedia or Last.fm, get lyrics from Google and more.

Album-Cover-Art.org - Allows you to search Album-Cover-Art.org for album cover artwork you may be missing for your music collection.

FindThatBand - Allows you to highlight a band name on a site, right click to select search, and it will pull up information about that band on MySpace, LastFM, Pandora, or Wikipedia.

FiTunes Sidebar Extension - Shows you a sidebar built around the top 25 iTunes Store categories such as singles and albums.

Harmony - This add-on integrates with Pandora or FoxyTunes to give you real-time information about the music you are listening to, and will also scrobble the songs to your Last.fm account for you.

Song.ly - Song.ly allows you to share songs with all of your Twitter followers with just two clicks, and will take them to a page where the song is embedded in a player for them to hear.  Sort of like Blip.fm, but it works more directly with Twitter.

TwittyTunes - From the same people who make FoxyTunes, TwittyTunes will allow you to post whatever music you have playing to your Twitter stream.  What is nice is that this isn’t done automatically, but only when you click the button, so you won’t end up spamming your followers with song title after song title.