Associate magnet links with Transmission in Firefox

http://ubuntuforums.org/showthread.php?t=308130
http://kb.mozillazine.org/Register_protocol#Linux_and_Mac

1) Type about:config into the address bar and press Enter.
* Right-click -> New -> Boolean -> Name: network.protocol-handler.external.magnet -> Value -> true
* Right-click -> New -> String -> Name: network.protocol-handler.app.magnet -> Value -> /usr/local/bin/transmission
* Ensure network.protocol-handler.expose-all is set to true

edit: Turns out Transmission supports DHT but not magnet links yet. Maybe Transmission 1.8 will.