Hmmpff… This is going to be the first time I see a case where reinstalling Thunderbird might be worth a try. Have you tried that?

Thanks for your response, David.

I just closed Thunderbird, downloaded the current esr release, replaced the existing Thunderbird app with it, and re-opened Thunderbird. The symptoms remain.

My profiles.ini file shows two profiles although I have only one (I just deleted a beta profile). My profile shows "locked=1" even when Thunderbird is closed. Is that appropriate?

Check the prefs.js file in profile. This seems some form of corruption, possibly antivirus interruption or file in read-only mode. Also, check to see if there are multiple versions, such as prefs-1.js, prefs-2.js.

Macintosh, no user-controlled anti-virus app, only what Apple does in macOS.

There is only one prefs file. It is not locked. I have read and write access.

Should I restore my prefs file from a back-up?

What I read about "locked=1" indicates that it is normal.

Here's what has my attention: profiles.ini and installs.ini each show three installations although I have only one. In each case, two of the three installations point to the same profile.

I don't know when these symptoms first appeared. I wonder if it has anything to do with my downloading a beta release yesterday (and deleting it today). My main installation is the latest esr release.

What's weird here, and the reason I thought reinstalling might work in this particular case, is that it looks as if some form of corruption was present in the application itself, not in the user data…

I'm not at the computer right now and cannot give you more specific instructions, but this could be a missing font or some similar issue. Try restarting the Mac in Safe Mode and see whether the problem happens there as well.