Fire.FM adds a Last.FM toolbar to Firefox
The Fire.FM toolbar is compatible with Firefox 3 and higher. It lets you create a new station by entering an artist name, see a list of similar bands or musicians, and access a list of recent stations. You can also use the toolbar to adjust the volume or skip tracks. And if you login to your Last.FM account and grant the toolbar access, Fire.FM will also let you tag and "love" tracks.
You can also hide the toolbar and it will keep playing in the background.
Last.FM is free for users within the US, the UK, and Germany. Outisde of those regions, it costs €3/month to use the service.