Earlier quoted context omitted.
Apologies if this is elementary, but have you tried turning off DoH? https://support.mozilla.org/en-US/kb/dns-over-https#w_off
I see glacial DNS resolution regularly when hitting the AWS authorization page with DoH disabled on my company's VPN. Resolves instantly in Chrome.
EDIT: I do not know why its an issue with firefox and not chrome, it's likely QUIC fucking up since it cant fragment and needs to fall back to TCP, chrome is probably error handling this better... dropping the MTU that low will make the fallback explicit: https://blog.apnic.net/2019/03/04/a-quick-look-at-quic/
EDIT2: Could also try disabling QUIC, instructions here: https://developers.cloudflare.com/cloudflare-one/policies/ga...