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+## 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`