Tixati bittorrent client
Tixati is another that is installed using a download
Look for the tixati download page and download the version for your arch.
http://www.tixati.com/download/linux.html
They don't have a repository, so you should return sometimes to check for any updates.
When a magnet link is not handled by tixati use this:
Edit the tixati.desktop file with a text editor and replace "%F" with "%U" in the line that starts with "Exec="
You can find tixati.desktop in /usr/share/applications/ or ~/.local/share/applications if you are using the portable version.
If that doesn't help, try adding association manually, in a terminal:
xdg-mime default tixati.desktop x-scheme-handler/magnet
If Firefox still doesn't underrstand to use Tixati, just look for tixati in /urs/bin and point firefox to that location. And let firefox REMEMBER this choice.