If you are uncertain that McAfee is the cause, you can test whether something started during Thunderbird or Windows startup is involved:
* Start MS-Windows in safe mode
* With Windows in safe mode, start [https://support.mozilla.org/en-US/kb/safetroubleshoot-mode|-thunderbird Thunderbird in safe modeTroubleshoot Mode].
If the problem is gone then you must dig to find which Windows installed program or Thunderbird addon is causing the problem.