Odd I am having the opposite problem on one site in particular. I can't open with .torrent, firefox fails to find tixati.
Anyway in firefox tools/options/ applications/ look for "magnet" change to tixati always. Just enter it in the search at the top of the options box. If it is not on the list right click a magnetic link and choose save as. A dialog window will open then find tixati and in the lower left of the box check do this automatically.
