This question has been asked many times over the years.

All computers are designed to have basic levels of security after booting up. They are called 'User Accounts'. Usually, the person who owns the computer sets up the 'User Accounts' and will set the first one with admin priviledges. All other User Accounts are usually standard types. Each person should have their own 'User Account'.

All 'User Accounts' can be accessed via a password, which grants access to that users desktop. Thunderbird can run in any User Account desktop. By default, Thunderbird stores profiles within each User Account.

This means you need to know the password of the User Account before you even get access to the desktop, so you cannot get access to see any emails if you do not know that password.

Thunderbird does provide a 'Master Password' option, but this is really designed as a protection level for passwords that are used to access servers, just in case they leave a computer with full access to anyone whilst they are not present.

You should signout of your User Account if away from computer to provide privacy, not just for emails, but for any of your documents.

Thanks, & could you recommend any extension or 3rd party program to "secure" or password protect just Thunderbird? I already have all user accounts password protected on this laptop. The reason I'm trying to password protect is that if/ when I would ever need to take computer for repair or whatever, I don't want anyone to access Thunderbird.

Well to be honest, I would backup/copy the necessary files and remove anything from a computer that could be perceived as 'personal' before allowing anyone to do any 'fixing'.

You would be amazed how many times I've been told - tech fixed my computer and now I've lost all my emails and contacts. Tech people are not always as savvy as you expect and will not backup inless you ask them to do it, which of course means you also have to pay for a new external hardrive for that backup.

So it is easier for you to backup/make a copy of the Thunderbird folder that holds all that users profiles. Exit Thunderbird before making the backup. It's default location is: C:\Users\<Windows user name>\AppData\Roaming\Thunderbird

Once you have got a good copy on external hardrive then you can delete it from your Roaming folder. When you get computer back, before you run Thunderbird - which you may need to reinstall depending upon what the tech does to 'fix' computer - copy the 'Thunderbird' folder back into the 'Roaming' folder first.