What is DOH?

DNS over https

Mace2 said

I have found that DOH does not work through VPN like Protonvpn. why should a TLS connection be affected by a VPN?

Do you mean that Firefox doesn't use DoH when you are on your VPN, or that your DNS queries are all failing if DoH is enabled (with the strict setting of network.trr.mode=3, i.e., no fallback)?

Correct. When I use Protonmail VPN DOH does not work when I go to the 1.1.1.1/help site for verification