have you checked the firewall on your internet security suite allows Thunderbird 60. Some like Norton simply block the new version as they don't recognize the version number,

I examined Windows firewall, and could find no reference to Thunderbird. Am using Malwarebytes, ditto.

https://support.malwarebytes.com/docs/DOC-1130

Use the instructions to create an exception for Thunderbird and see if it helps.

I am guessing now, but open the error console (Ctrl+Shift+J) and clear it. Click get mail. What errors get picked up? I am wondering if we might have a mail server secured by a disallowed symantec SSL certificate.

T. exception established; no change.

Error console cleared; no change (stays clear).

I'll try re-installing T. If that doesn't work, will just live with it. Thanks for your help. (I wasn't aware of the error console!)

eddiejoepopcorn said

(I wasn't aware of the error console!)

Treat it with a little contempt. it fills fast with all errors and most of them mean nothing. Other than you opened a mail or an RSS feed with unsupported HTML or CSS. Hence the suggestion to clear it.

Further things you can do include;

• 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.