Well, this is odd. Somehow the footnotes are not rendered correctly. At first I thought I forgot to send them. But I did. Trying to edit the post, I can see the footnotes are present:
That is correct formatting. I don’t want to pick on Discourse again, but I really need to understand what I need to change in my settings to make this work, but also keep it workable for me. Breaking lines at between 72 and 80 characters is/was quite common among e-mail clients.
In a smaller, narrower window it still looks correct:
Anyway, I reset mail.wrap_long_lines and plain_text.wrap_long_lines to true, even though that feels like doing the opposite. But these were the only settings related to wrapping, that I had modified.
If that doesn’t help, I will need to find some Thunderbird guru, who can tell me about what black magic I need to perform.