• Start Windows in safe mode with networking enabled - https://support.microsoft.com/en-us/help/12376/windows-10-start-your-pc-in-safe-mode
• Still In Windows safe mode, start Thunderbird in Troubleshoot Mode - https://support.mozilla.org/en-US/kb/troubleshoot-mode-thunderbird

Does problem go away?

Thanks for the super fast response! I really appreciate that.

I started Thunderbird in troubleshooting mode without putting Windows in Safe mode. It seems to work OK. Thinking back, I have only installed one add-on to Thunderbird ever. That was ImportExportTools NG. It worked perfectly in the previous versions. I have disabled it and I will watch over the next few days to see if disabling it solved the problem.

Do you have v. 14.1.2 of IET NG? It works fine here in TB 128. There's probably something else affecting performance, such as hardware acceleration (run in normal mode, disable in Settings/General, restart TB), or broken layout (launch in safe mode, 'reset toolbars and controls', 'make changes & restart').

http://kb.mozillazine.org/Safe_Mode

Thanks for the response!

I have ImportExportTools NG 14.1.2 installed and disabled. As Thunderbird is working perfectly fine with it disabled and since I rarely use it, I will leave it disabled until I need it in the future.

I sent 4 test messages from a web based email to each of my accounts in Thunderbird. After each message I tested them and I encountered no freezing issues.

As it is working correctly I will leave all settings as they are.

Again, thanks for the suggestion and your prompt response! I appreciate the excellent help I always find in this forum.

Sometimes, launching just once in safe mode and making no changes corrects issues that have no known source, as if safe mode resets some elements to make normal mode work correctly.