I think you have misunderstood the purpose of the master password: it is not to keep your emails private, it is simply to protect your email passwords from others who may use your machine. Read here: https://support.mozilla.org/en-US/kb/master-password If you want to prevent others from seeing your mailboxes the best way is the 'user account' feature that is part of your operating system. If you log in with a password when you start your computer and log out when you finish (or leave the machine for a while), then others cannot see any of your data.

No matter what the intention may have been, the implementation is poor. When I start Thunderbird and a password is requested, it should be requested before any other action i.e. before mailboxes or anything else is displayed. I share my machine with others in all other aspects except e-mail. I cannot have my own user account for good reasons I cannot explain, but I would nevertheless like to prevent access to my e-mails and I thought a well implemented password feature would be a solution.

Thunderbird master password protects the password store only, as intended. Nothing more. Master PW never will protect the local storage or viewing capabilities against the local storage.

Account passwords protect your account data at the server - not at the client.

PC passwords protect the data on on your PC and the ability to run software that accesses that data, including Thunderbird. As does disk encryption. THAT is where you should be investing your protection efforts and dollars.

For the multiple prompt problem see https://addons.mozilla.org/en-US/thunderbird/addon/startupmaster/?src=cb-dl-mostpopular

> I share my machine with others in all other aspects

They should have their own accounts on that machine and not be administrators. Then they can't see your data.

http://kb.mozillazine.org/Protect_the_profiles_contents