Primary Password with start up prevention

Hell will freeze first. See this: https://bugzilla.mozilla.org/show_bug.cgi?id=1566458

Your only true safe approach is to password protect the PC or set up different PC profiles for each user. Even if the user was knocked of thunderbird the messages are still accessible via file explorer.

Not AFAIK, as feature of Tbird (don't know why). Maybe some mozilla addon does this? Or 3rd party app. Technically, if someone w/ any computer savvy can access a device's storage (locally, remotely), they can grab stored email or data, w/out opening the client (e.g., Tbird), then read it somewhere else.

Not an expert here, but only way I've read to prevent this, is to encrypt the email data, so it has to be decrypted w/ a PW before can be read (whether data is copied at the device, or DL remotely by hackers).

Often requires creating a separate partition. Possibly other "container" to encrypt. Commonly done in Linux distros. Several acclaimed, open source encryption apps (for 'Nix). Maybe for Win.

Data partition often has to be decrypted, before the app using the data is launched, so data is readable, where it's expected. Or may be able to put the app (Tbird) & data on same encrypted partition & decrypt both at same time. Lots of people encrypt their entire 'home' partition (in 'Nix) where many of user's personal files, data are stored.

It's not that hard (in Linux). Probably possible in Win, but I haven't used it in years.