Why would you choose cancel? If you do this you can't send or retrieve any new messages.

The StartupMaster addon will help you by asking you for the Master Password only once. If you choose cancel, it can be set to either close Thunderbird or just continue in its non-connected mode.

1P0D1Alexandr said

I have set up 4 mailboxes in thunderbird. I also set masterpassword. When I start the program, if I do not enter a masterpassword, I must click cancel 4 times (for each mailbox).

This is really ANNOYING! I have this problem too. No need to enter the MASTER PASSWORD 4 times (i also have 4 accounts!). ONE is enough, and then the dialogs can be ESCaped.

Now... i finally resolved this issue, by installing this great ADD-ON: https://addons.mozilla.org/pl/thunderbird/addon/master-password/

You can have total control of the master password (avoid TB loading, request password after a certain time, etc etc). A must have!