Thunderbird "undo send" feature (like in Gmail)?

That add-on probably is what you want actually, as that is all Gmail's "undo send" option really does behind the scenes. It doesn't actually send your email's instantly, but rather delays them so you have the option to opt out before it's actually sent. The option doesn't actually dig into people's mailboxes and take the email out. Once it's sent, it's pushed to a different mail server. If everyone in the world used Gmail, maybe they could take it out of someone else's mailbox for you, but since everyone doesn't, this is how their "undo" feature actually works.

Try the "MagicSLR" extension. You can add a "Send Later button" or use it to replace the Send button.

MagicSLR extends functions and buttons to get and send messages. Many users missed a combined "Get/Send" button in Thunderbird. MagicSLR provides this button and more.

I am not sure if this is relevant but I stumbled upon the ability to cancel a message after I have hit the "Send" button. Usually there is a small window of opportunity after hitting "Send" in Thunderbird when Thunderbird is busy processing your request. During this brief few seconds, pressing Ctrl + Alt + Enter will actually cancel the sending of the message. Of course, larger e-mails ie. with attachments, afford you a bit more time to cancel your e-mail. I find this helpful if I have realized a typo or forgot to add content, I can stop the send process and go back and edit it.