Thunderbird: Confirm encryption absence/presence before sending an e-mail
Since OpenPGP-Integration in Thunderbird 78+, Enigmail is deprecated in Thunderbird and a confirm dialog is currently not integrated in modern Thunderbird versions.
To avoid sending e-mails containing sensitive data unencrypted I strongly suggest you enable a confirmation dialog.
Enigmail can show if the e-mail being sent will be encrypted or not -- and if, which keys will be used. This applies to both clicking the "Send" button or pressing Ctrl+Enter. Thus, you may disable the confirmation for the keyboard shortcut as you get a message box anyways which will avoid sending e-mails by accident.
Instructions
Head over to the Enigmail preferences from the "OpenPGP" menu and tick the corresponding checkbox.