That is one way to handle a Firejail sandboxed browser, but I do not think that changing "mount points" is necessarily a good thing to do for this, and certainly not for a new or inexperienced user. Perhaps you are using "mount points" to mean something else than what it usually means.
Another way is to just use your "Downloads" folder for storing the torrents information files and the actual file downloads, and change the torrent software client to look in those folders. For instance, you could create a "/Downloads/torrents/NewTorrents" folder for saving ".torrent" information files and a "/Downloads/torrents" folder for saving the actual downloaded files (like a "LinuxMint.iso" file) and use those.
In the Torrent software clients, you can just open the saved ".torrent" information file from the Torrent client software to start the download, some Torrent software clients, like Tixati can be set to automatically start downloads from torrents saved in a folder you set, like "/Downloads/torrents/NewTorrents", regardless whether the browser is "firejailed".
+1 And of course, you can also add a whitelist entry to Firejail for browsers and torrent software client profiles for torrent folders.
FYI-1: The Firejail sandboxing with browsers also affects other downloads, like Linux software installation files. Best to save the installation files into your "Downloads" folder, and run the installation from outside of the Firejailed browser, even if the browser automatically starts the installation file (like a ".deb" file), then close it, bring up your file manager, and browse to your downloaded file and double click it to install it.
FYI-2: When I am using a Firejailed browser, I cannot even upload screenshot pictures from my "Pictures" folder. So, I created a ScreenShots folder underneath my "/Downloads" folder for that too, "/Downloads/ScreenShots", and I save my screenshots into that folder.
Hope this helps ...
"Tixati" Torrent software client
Location Folder Settings for actual downloaded files from torrents
Location Folder Settings for ".torrent" files
"Ktorrent" Torrent software client
Location Folder Settings
Opera browser with Firejail - saving a Linux Mint .torrent information file
Example of simply opening a ".torrent" file from within a torrent software client (Ktorrent) to download the file, in this case a Linux Mint .iso file.