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.

Clicked on "Restart with Add-ons Disabled" and nothing happened. I have not knowingly installed any Add-ons". I did run Thunderbird.exe -p to set up multiple inboxes. Was this an Add-on?

No that's not an add-on. It is an unnecessarily complicated way of managing email, IMHO. I just add all my accounts to one single profile.

You may have the Lightning calendar installed, as it comes by default. Disabling it would be useful as it has caused particular problems of late. But if "Restart with Add-ons Disabled" hasn't improved the situation, we can rule the calendar out.

Yet you said "nothing happened". Do you mean it didn't re-start, or that it ran and still didn't work?

Holding down shift whilst re-starting Thunderbird is another way to run in safe mode, that is, with all add-ons disabled.

Clicked on "Restart with Add-ons Disabled" and it did not restart. Holding down shift while restarting did not enter safe mode. I do not as yet have any stored emails. Is uninstalling and reinstalling Thunderbird my best option? If I reinstall, is there a way to make Thunderbird group emails by email address if I don't set up separate inboxes?

open the task manager (alt+Ctrl+Delete) and check the process list for Thunderbird after you close it. If it is there terminate the process.

Now try holding the shift key and clicking Thunderbird to start up. I am guessing whatever is happening is hanging the application after it takes down the last window. Hence the task manager to end it.

I opened the task manager. Did not find Thunderbird. I tried safe mode (shift / thunderbird). This time it worked and Thunderbird safe mode worked. Found and disabled extension Lightning and Thunderbird seems to be working. Thanks