auralov, I think that the only available protection method offered by Firefox is to use a Primary Password. See: https://support.mozilla.org/en-US/kb/use-primary-password-protect-stored-logins

Note that once you enter the PP during a session that the logins remain unlocked and you can see the usernames in about:logins and they can be filled on a webpage. Viewing a password requires to enter the PP, canceling a PP prompt will re-lock the Password Manager and about:logins.

• https://support.mozilla.org/en-US/kb/password-manager-remember-delete-edit-logins