For peers to connect to each other, at least one of the two connecting needs to be port-forwarded, so if everyone were on Mullvad, now no one would be able to reach each other. Right now you can still torrent on Mullvad, but only with (non-Mullvad) users who use port-forwarding.
LibreWolf is so clean and minimal, whenever I go back to Firefox it feels bloated in comparison.