It would be great if Thunderbird could offer the ability to undo the sending of an email for a few seconds after the actual sending, similar to what GMAIL does with the "undo send" feature.
This feature has saved my life several times over the past decade. Switching back from Gmail without this kind of feature would be a real step backwards for my sanity :-). Unfortunately, Thunderbird does not even have the ability to apply filters to outgoing messages, so it is not even possible to delay sending all messages as can be done with competing email clients (Outlook, for example).