Have you considered running a SOCKS proxy outside of Mullvad (ie on a Raspberry Pi or in the cloud?) You could then use Firefox Multi-Account Containers to bin

Have you considered running a SOCKS proxy outside of Mullvad (ie on a Raspberry Pi or in the cloud?) You could then use Firefox Multi-Account Containers to bind a container to the SOCKS proxy, and whenever you need to access a site that doesn’t support a VPN you can just open it with in said container.