What do you mean? Where is it FF supposedly saving it (except if in some temporary path of his)? As far as I know - you should regard this as temporary (like some other internet cached data) while the "real" .torrent file is being saved by the client (qBT/uT) in the designated path you defined for it's .torrent files.
Edit:
I see what you mean, FF saves it in the downloads folder. Open a bug-report with FF Bugzilla...
The W/A is to set in about:config -> browser.download.improvements_to_download_panel = false , with the hope that they will change the behavior in the future... Looks like you are not he only one that is not happy with this change... https://support.mozilla.org/en-US/questions/1370409