What is your anti-virus software?

worked fine with System Mechanic software/antivirus program until latest "Thunderbird" update--again, I am using Windows 10 64 bit

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?

chiefholt said

worked fine with System Mechanic software/antivirus program until latest "Thunderbird" update--again, I am using Windows 10 64 bit

This is the equivalent of saying my car ran fine on gasoline until it didn't. Maybe you have water in your gas.

The majority of lockups and slow performance is because of antivirus issues. If your AV software does not update maybe you need a different one.