Since The Pirate Bay dropped torrent files and started only providing magnet links the other day it became quickly annoying that Firefox wouldn't open them or associate them with my torrent client. The error message one encounters is:
Firefox doesn't know how to open this address, because the protocol (magnet) isn't associated with any program
The solution is to open about:config and add this key:
network.protocol-handler.expose.magnet (boolean) false
Now when you click on a magnet: link you will be prompted to select an application to open it with. In my case I added /usr/bin/vuze and checked the remember box.