Safe mode disables addons, so I would guess you have one that is not doing what it says it does.

About the only other "common" thing that is changed is the hardware acceleration in settings is disabled and this, when it goes wrong, offers some of the weirdest symptoms. So you might check it is actually disabled in normal mode to exclude it as a cause.

Thank you for your response. I un-checked acceleration, but it had no effect. When I open the Add-ons Manager (not in troubleshoot mode) I don't see the five add-ons I know I have, so I can't selectively disable them. The ones I have probably all came with the upgrade (I can't keep track of which upgrades). They are Google, Amazon, Wikipedia, Duckduckgo and Bing. So I'm still having to use my Thunderbird in safe mode, and any time I turn off my computer or close Thunderbird I have to switch to troubleshoot mode again. Otherwise no content in any of my in-boxes or files. Any other ideas? Should I try upgrading again? Thanks, Matt.

I am struggling here. Trying to think of what else is really covered by the use of troubleshooting mode. One is Third party modules. (usually antivirus products) Go to the troubleshooting information on the help menu. (in normal mode) click on the About:third party modules entry. Is anything shown?

It is good of you to work at this. No, there are no third party modules detected (in normal mode). I don't mind operating in troubleshooting mode. And I do a zoom every week with a friend who is also helping me to solve this. Perhaps my next step is to try an upgrade.