To diagnose problems with Thunderbird, try one of the following:

• 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.
• Restart the operating system in safe mode with Networking. This loads only the very basics needed to start your computer while enabling an Internet connection. Click on your operating system for instructions on how to start in safe mode: Windows 10, Windows 8, Windows 7, Windows Vista, Windows XP, OSX

If safe mode for the operating system fixes the issue, there's other software in your computer that's causing problems. Possibilities include but not limited to: AV scanning, virus/malware, background downloads such as program updates.

Thanks for trying to help. No change in Thunderbird safe mode. Windows 10 safe mode was worse - now when I open Thunderbird I get an error dialog: "Failed to connect to server ...".

Update: I found my firewall was blocking Thunderbird after doing the safe mode reboot. Now the first time (and only the first time) I open Thunderbird after a restart I see a certificate error dialog (see attached). If I click on the Confirm... button at the bottom, the dialog closes. The status message at the bottom is still "Connected to..." and no new email appears (even though I can see new email via webmail). If I click on the "Get Certificate" button, the dialog display changes to show "Unable to obtain identification status for this site."

Found the problem: Bitdefender was blocking Thunderbird from getting an SSL certificate from my host server. Had to disable the "Online Threat" called "Encrypted web scan" in Bitdefender.