How to reassociate torrent download in Firefox to another torrent program

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http://askubuntu.com – For now my girlfriend is using Transmission as the torrent downloader. She downloads magnet links and they start after a couple of seconds to download the content. Today she wants to try qbittorrent. Firefox does not show an option when downloading the magnet link, it just starts transmission assuming that it is the one she wants. She downloaded qbittorrent and thought that firefox would show an option to download either from transmission or qbittorrent. Since there is no such option, how can it be reassociated or replaced the transmission for qbittorrent in Firefox so that when she downloads (HowTos)