This question comes up frequently. A [[Protect your Thunderbird passwords with a Master Password|Master Password]] in Thunderbird is not meant to protect your mailbox content from viewing, but only from accessing stored server passwords for sending and receiving that you don’t want others to view. If you want to protect your Inbox from other people using your computer, let them use a guest account in Windows. Even if you could protect the mailbox content, any user that has access to your system can access its mail folders. As for opening Thunderbird and having it set to select a specific folder, you could try the solution with the ''-mail'' startup parameter mentioned [http://kb.mozillazine.org/Show_Inbox_when_starting_Thunderbird here]. Note that the add-ons mentioned there no longer appear to be maintained or work, but there may be others that are/do.