Hi element, first use the extension to define the Content-Type you want to use for the specific file extension. Then when Firefox shows the dialog to Open or Save, use the drop-down next to Open with to select the application you want to open the file.

If the torrents do not have a standard file extension, then a different method would be needed. You could post a question on Mozilla Support (https://support.mozilla.org/questions/new/desktop) or on Reddit (https://www.reddit.com/r/firefox/) for more specific advice.