I am not sure what we have there, but you have at least 4 tabs open two for inboxes and what appear to be two that have not yet complted loading. What happens if you go get coffee an wait for the startup or whatever is causing the not responding to time out. Then you might get an error message of some sort of offer a clue as to the cause of the problem.

Thanks for the suggestion, but within 3 minutes Windows gives the "Thunderbird not responding ..." message as attached. Even if you click the "wait" after another 5 minutes you get the same message and choice. So no clue appears from Tbird.

I think I need to replace the profile from a backup. Any suggestions there?

Sorry for not responding earlier in https://support.mozilla.org/en-US/questions/1496153 - the support system's notification process hasn't worked for a few month, so I never saw your response there.

FWIW, in the last 8 months many bugs have been reported with these symptoms, but so far I think only one has been identified as coming from Thunderbird code - do you have all or mostly pop accounts?

Also, does the problem change with Windows started in safe mode with networking enabled - https://support.microsoft.com/en-us/help/12376/windows-10-start-your-pc-in-safe-mode ?

jfcasey ??