Do you need to add the column for the sender's names?

Look for the tiny button (see attached), or right-click on any of the visible column header labels.

There's been no deliberate change to Thunderbird's appearance (in this particular respect) but it does seem that on an upgrade some settings change unbidden.

I see the problem now. I can add the From column and see the senders name. I had installed Thunder Conversations add-on, which has a Between column. That column was blank. I reinstalled the add-on. It is now working correctly.

Thanks for your help.