try clicking on the little square box on the heading line and turning on a heading. Say subject. Anything displayed?

Yes, that solves in part!. I can't figure out now how to get the subject field to display. Why isn't that in the list?

I don't see subject in that field list for other folders that have a subject field either.

I see it is missing from mine as well. I guess someone removed something.

Goto the trouble shooting information on the help menu. Click on the show profile button. Close Thunderbird. Delete the file session.json in the profile folder and restart Thunderbird. That should force a return to defaults.

Unless it is caused by an addon or theme issue. Restart Thunderbird with add-ons disabled (Thunderbird Safe Mode). On the Help menu, click on "Restart with Add-ons Disabled". If Thunderbird works like normal, there is an Add-on or Theme interfering with normal operations. You will need to re-enable add-ons one at a time until you locate the offender.

Starting in safe mode didn't change anything re: subject field, nor did deleting session.json. This is maddening. Thanks much for continuing to help.

Open a folder that has the column titles and messages properly displayed, then click the column selector icon at the right end of the titles, Apply columns to... and select the faulty folder.

thanks sfhowes, that solved it. I don't know how this happened, so it doesn't happen again, but oh well. Cheers!

Re Matt's contribution. After closing Thunderbird as suggested, where exactly do I find the "the file session.json in the profile folder" so it can be deleted?