if you want local security, windows supplied a very good user account system. If you upgrade to windows 10 you will find yourself putting in your password every time you want to use the device. See https://support.microsoft.com/en-us/help/17197/windows-10-set-up-accounts

What Thunderbird has is stored on your personal computer, so signing out of it is no more possible than it is to sign out of Microsoft word installed on the local machine.

Thanks for your quick reply. I have a laptop computer & running Windows 7 (32-bit). I'm I understanding you say there is no way "hide" my messages in Thunderbird? Please, could you supply more info. I can give you most any info you might need to answer my question.

Thank you

I really do not know how to answer a question for someone that refuses to accept no as an answer..

Look try this add-on. It will do what you want. But is about a secure as a wet paper bag. https://freeshell.de//~kaosmos/profilepassword-en.html

You do not sign into Thunderbird so why do you think that you could sign out?

You provide a password when your email providers server asks for one to deliver or send your mail. Once again you do not sign out of your email account.

As Matt suggested use the security built into your operating system to prevent people viewing what is on your computer when you are not around.