What is your anti-virus software?

AVG Free

Try to start Windows in safe mode with networking enabled. - Win10 http://windows.microsoft.com/en-us/windows-10/change-startup-settings-in-windows-10#v1h=tab01

Still in Windows safe mode, start Thunderbird in safe mode. - https://support.mozilla.org/en-US/kb/safe-mode

Does the problem go away?

Before I do this can you please tell me what good it would do? I don't usually operate my PC in safe mode. What does it change by doing this? Thanks.

It disables certain software which is started automatically in normal mode, such as anti-virus software. The idea is to verify whether anti-virus software is messing with Thunderbird.

I tried what you suggested and it appears to work fine in Safe Mode. Now what do I do?

Windows safe mode disables anti-virus software, which is most likely the culprit.

Create an exception in your anti-virus software for the Thunderbird profile folder, so that the anti-virus real-time scanner will not scan it. http://kb.mozillazine.org/Profile_folder_-_Thunderbird

Don't let your anti-virus software scan incoming and outgoing messages.

Don't let your anti-virus software scan attachments.

Don't let your anti-virus software intercept your secure connection to the server.

Remove any add-ons your anti-virus software may have installed in Thunderbird.

I would prefer it if Mozilla was able to figure out the problem and fix it from their end so that I wouldn't have to jump through all of these hoops. Why did Thunderbird work perfectly fine with Windows 7? I was using the same anti-virus software then.

The problem isn't caused by Thunderbird but by anti-virus software messing with Thunderbird mail files. Feel free to complain to AVG. You're the one with the problem. You asked a question, and you got an answer. I'm sorry if you don't like the answer. Good luck.

OK. I'll try that approach. Wish me luck. Thanks for your help.

I'm back with the same problem. I uninstalled AVG free and am relying on Windows Defender as my anti-virus program instead. But I am still having the "freeze" issues that I had before. What to do now?

What to do now?

Same as before. See https://support.mozilla.org/en-US/questions/1114834#answer-858839

The link that you provided shows me how to locate my Thunderbird "profile" folder. However, I'm still not clear how to make my anti-virus software make an exception or allow the email to work. A step-by-step process would be more useful than the link that you provided. Sorry.

Please contact Microsoft or ask in a Microsoft forum.