Have you restarted (rebooted) your device? If not do so.

Thanks, but yes, first thing I tried. Actually, I wound up rebooting twice. I followed the advice for another Firefox problem thinking it may help Thunderbird as well, so I had to reboot for that too. Still no good. I was eager to NOT lose my saved emails so I hesitated uninstalling Thunderbird and then reinstalling. Instead, I decided to try 'repairing' the app by simply downloading a new copy without uninstalling the current one first. It went through the normal paces and ran the installer wizard. All of a sudden, the program opened and all is well... everything saved is still there. I don't have 2 Thunderbird installations, as far as I can tell, but it apparently worked to download a 'fresh' copy.... whatever, it worked! So, issue RESOLVED! Thank you so much!