Amarok has been able to play last.fm streams for a while now, but I was looking for a way to click on a link in firefox, and have it play straight away in amarok, no cocking about. I searched for a while to get lastfm:// links opened in amarok from firefox properly, and eventually found it in a "how to do this in opera". Heres how:
This can also be used to add other protocols, mailto: for instance. Use the same method, but change the last bit to suit your protocol, eg: network.protocol-handler.app.ass for ass: links (common on government sites*)
*Not really