More information. Thunderbird is started automagically when the computer boots up, I presume that is when the update was installed.

In the pop mailbox window a message when clicked on is highlighted. Prior to the update the message body would be displayed and after a minute (by preference) the message subject line would change to plain to indicate it was read - that no longer happens in addition to not displaying the message body. If I select Reply from the drop down, no to address is shown regardless of which flavor of Reply I selected. No themes are installed. The only extension is "Allow HTML temp"; disabling it made no difference. Looking in Library/Thunderbird/Crash Reports, the newest file is InstallTime20221219215418 which contains 1677084866. Looking at the file modified times in my profile /Users/rod/Library/Thunderbird/Profiles/ix3tiltd.default I see that Thunderbird was started at 08:54 and at 08:55 the directory crashes was updated and contains store.json.mozlz4 which I was unable to attach. Let me know what other info you need. FWIW that inbox is very large with 11K+ messages.

The IMAP account works as before. None of what I note above occurs for that account.

Quit and restarted Thunderbird. The message view problem appears to be gone. But there was some sort of first run after update install problem at least for me, possibly for others, and for other updates.