I replied to a similar query yesterday and some of what I wrote then may apply to you too:

https://support.mozilla.org/en-US/questions/1034630

Specifically for protecting the entire profile, you may find the StartupMaster extension useful:

https://addons.mozilla.org/en-US/thunderbird/addon/startupmaster/

It may not be obvious from the description, but when installed, the first thing you see is a login password prompt, and only when that's entered correctly does the main Thunderbird window appear.

Bear in mind that since it's implemented by an add-on, a savvy user can easily circumvent the password challenge, and in any case, your emails are stored in plain text and may be discovered by anyone browsing through your file system.

The designers of Thunderbird chose not to re-invent the wheel; if you are concerned about prvacy, they assumed users would use the tools built into the operating system; user accounts, logins, passwords and encrypted file systems.

The 'Master Password' is designed to protect prying eyes from seeing the stored passwords in Thunderbird and nothing else.

To stop people seeing any of your documents on your computer you should set up User Accounts.

'User Accounts' is the standard method of protecting your privacy. Do this via the Control Panel. Set your User Account as admin if it is the first User Account on your computer. you can create other 'User Accounts' for different people if you are sharing the computer. You can also create a 'Guest' User Account' with no password for other casual users.

The User Account is set up to use a pasword. When you boot up computer, you have to access the User account using your chosen password. It will then load your desktop.

Thunderbird will run in any User Account. Thunderbird keeps all Profile info including emails etc in the User Account. So if you do not know the pasword to the User Account then you cannot get access.