Firefox doesn't know how to open this address,
because the protocol (SOMETHING)
isn't associated with any program

magnet: links were not working in Firefox and it finally reached the point where I felt that it deserved fixing. Like most of the things I postpone, I realized I should’ve just done it sooner. Would’ve saved me the agony. I will quickly describe what to do when you want to add the handler for a magnet link, but the procedure is the same for other protocols. Simply replace occurrences of magnet.

Go to Firefox’ good old about:config and search for network.protocol-handler, then check if the following two settings show. If they do not, add each by right-clicking and selecting New > Boolean.

network.protocol-handler.expose.magnet

Set this one to false.

network.protocol-handler.external.magnet

And this one to true.

Now next time you click a magnet link, a dialog will open up asking you how to open the link. Go to your favourite torrent client (Transmission, Deluge, rtorrent, …) and select it. Example for Deluge on Linux: /usr/bin/deluge. You can make the association permanent (so it does not ask you for confirmation every time), by going to Preferences → Applications and selecting your choice.