When you click a link to a .torrent file in Firefox, after installing Folx for the first time, a dialog will pop up prompting you to select what to do with this file.
If you want to download torrent contents with Folx:
Select Folx from “Open with:“ drop-down menu, tick “Do this automatically for files like this from now on” checkbox and click “OK”. Firefox will download .torrent file. After download is complete this .torrent file will be opened by Folx and Folx will start downloading the torrent contents.
At some torrent trackers “Do this automatically for files like this from now on” option might not work as it should work after enabling it. Each time when you download torrents from these specific torrent trackers, Firefox will show opening dialog prompting you to confirm opening of .torrent files with Folx.
This is not the issue of Folx, this is the peculiarity of how Firefox works with some torrent trackers.