Proton emails are stored in an encrypted form that goes beyond the simple authentication that is part of the POP/IMAP specifications

Proton does have open-source bridges/proxies, so they aren’t hiding these details from us

Perhaps Thunderbird could be enhanced to support the Proton features directly?

https://thenib.com/mister-gotcha/

EFF still recommend Signal (and others) for people fitting various risk profiles: https://ssd.eff.org/

Google is also going with a combined approach: https://security.googleblog.com/2023/08/toward-quantum-resilient-security-keys.html