Are you talking about the send later shortcut perhaps?

Send now is Ctrl+Enter and send later is Ctrl+Shift+Enter

There is no button for send later as such, but send becomes a send later button if you place Thunderbird in offline mode. (click the ((0)) at the bottom of the main screen on the left.)

No, I'm just talking about the Send button in the upper-left corner of the Write window, which causes the email to be sent immediately. I sometimes click it by accident. Once I do, I can't stop it from going. In fact, this is such a huge problem that with important emails, I add an X to the address so that the email won't go if I accidentally click on it, as follows: abcdefg.comx

It seems reasonable that I should be able to confirm the sending of an email, but the program doesn't seem to have such an option.

See https://addons.thunderbird.net/en-US/thunderbird/addon/check-before-sending-email/

https://addons.thunderbird.net/en-US/thunderbird/addon/check-and-send/

Thank you. Which one do you suggest that I install? I want the one that is easiest to use.

I didn't know that TB accepted add-ons.

I haven't used them, so I suggest trying each one, one at a time.

https://support.mozilla.org/en-US/kb/installing-addon-thunderbird

Thank you, but one of the add-ons is totally clunky, and the other is not compatible with my version of Thunderbird.

You might achieve the same or similar check with add-ons like Correct Identity or Identity Chooser.

What do they do?

What I did was I set TB to do a spell check before sending, but if there are no misspelled words in the email, it doesn't pause and give me a chance to cancel the Send.

Thank you, but one of the add-ons is totally clunky, and the other is not compatible with my version of Thunderbird.

You're using v115? Update to at least v128 and Check and Send will work.