It crashes or is not responding? That's not the same thing.

It is not responding which requires closing and then reopening the program.

What is your anti-virus software?

AVG Internet Security.

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?

Will give it a try, but being a novice at this business, this will take some time.

well you could uninstall AVG and see how that goes. I would guess it is scanning the newly created file and doing it badly. Hence the hang