This on Windows 7 OS platform.

Install this add-on. https://addons.mozilla.org/en-US/thunderbird/addon/startupmaster/

It is not a solution to the problem I have described . It is necessary to fix the program. And the use of third-party add-ons - a sign of complicity with the developers in a possible theft of personal data of users. You agree that the program should work without any add-ons !

Whether you use the add-on or not is entirely up to you. I have no idea what 'possible theft of personal data' you're talking about.