How to Prevent Mozilla Thunderbird from Inquiring About the Format When Sending

Let's have a little quiz: he consumes each year one and a half times his own weight in other... — Yes, John?
"Who is the healthy male adult bore?" — "Spot on!"
John Updike can complete the answer, his answer, too: "A healthy male adult bore consumes each year one and a half times his own weight in other people's patience."
Whether Mozilla Thunderbird is male or adult is not so clear, but its permanent asking for message formats sure can bore and consumer the user's time. Reclaim yours:
›› Does Mozilla Thunderbird ask you — all the time — whether you want a message you're about to send delivered in plain text or HTML? Here's how to stop the questioning and make Mozilla Thunderbird do what you choose — all the time, and automatically.