Thunderbird complains "Signing certificate authority is not known" suddenly

Yesterday, Thunderbird repeatedly returned an error over the TLS certificate. I can see in Firefox that mailbox.org's cert is signed by DigiCert yet Thunderbird reports a cert from Thawte. I have used mailbox.org with Thunderbird for years and not encountered this before.

It's not a DNS issue since only Thunderbird has this problem and the DNS service is trustworthy

Please help me diagnose and resolve this.