For reference, since I had never heard of "PEC", from Wikipedia: "Certified email (known as Posta elettronica certificata in Italy, or PEC in short) is a special type of email in use in Italy".

I have the same issue for incoming/outgoing S/MIME signed emails.

128.6.0esr (32-Bit) on Windows 10.

The issue is due to a recent regression: https://bugzilla.mozilla.org/show_bug.cgi?id=1940605. It has already been fixed; the fix is part of version 128.6.1esr. The release notes say that the new version should have been published already (Jan. 22), but there is no download yet: https://www.thunderbird.net/en-US/thunderbird/128.6.1esr/releasenotes/.