I suppose you're talking about the Primary Password. The Primary Password protects the account password(s) of the email accounts you set up in Thunderbird. It does not protect the contents of the Thunderbird profile. Also see https://support.mozilla.org/kb/protect-your-thunderbird-passwords-primary-password

thank you

It is as @Christ1 explained: it only protects the account passwords. So, another person may see messages on the PC that have been downloaded, but cannot send or retrieve mail. For account security, you need to pursue at a higher level, such as with a Windows password. This does not provide the security you want.

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