About Proton Mail Bridge integration
thunderbird user.js enforces certificate strict PKP (Public Key Pinning) verification. This breaks software performing MITM (such as a local antivirus), or even Proton Mail Bridge.
When connecting Thunderbird for the first time to Proton Mail Bridge, this hardening measure (1223) must be relaxed :
security.cert_pinning.enforcement_level to 1
Once incoming/outgoing servers enrollment is done, you may switch back to template value :
security.cert_pinning.enforcement_level to 2