How can I get Thunderbird to send in HTML?
I have since discovered that Thunderbird IS sending in HTML, it is just that I seemed to be unable to compose messages in HTML, although I could cut and paste HTML.

I have resolved this by enabling View/Toolbars/Formatting Bar (although I don't know WHY it wasn't enabled).

I would still like to be able to tell Thunderbird to use a format on a message by message basis, but can find no such option.