Okay, for the benefit of a very few, I've set my tbird configuration to
mailnews_send_plaintext_flowed = false;
Since I've got several accounts on this client and this is a *global*
composition setting, I'm going to hope that it doesn't mess up composing
on my other accounts. I've set the line width to 72 characters.
If it does, I'm going back to flowed sent text and you can simply delete
my messages when you see them.
Of course, there's always the chance that this option won't work at all
makes you happy, fine. Otherwise, I'm not willing to do much else.