Add ons I have installed on my Firefox are....
Adblock Plus....................for blocking Ads etc. handy
Fasterfox.........................do a few tweaks, speed things up
FireFTP............................for FTPing, obvious really.used to use WS_FTP this seems better
FoxyTunes........................the music one mentioned in the link.Like it

FoxyTunes Skin Aqua Bubbles.......and a skin for the above
Mouse Gestures..................'cos I liked using them in Maxthon and it didnt feel right without them in FF
Tab Mix Plus......................set the tab features just how you want.