I have used Thunderbird for more than a decade.

Okay, searching around I found how to turn on “troubleshooting mode”, and selected to turn off all add-ons.

I now get the actual email interface, with buttons, various inboxes and controls that were all gone from the application before.

Such a relief.

Now I wonder what “add-ons” were there, and how to get rid of them permanently. I would rather simply be able to use the email client again.

So, I tried what the instructions say, to disable an extension or “add-on” and see if that makes it better. The problem with this instruction, is when you start Thunderbird in troubleshooting mode, it takes away any extensions that may have been there, and gives no indication of what has been disabled.

I poked around and only see “System theme — auto” default-theme@mozilla.org

I found that perhaps the “troubleshooting mode” is persistent, since now I can close and open Thunderbird normally and the application seems to be working now.

I have doubts that it really is okay now, but will try to keep using it. I wonder if I should seek out a different email application in order to have more reliability.

Still I do not know what caused the problem.

Perhaps it is solved for now.