This is one of those "I need to write this down because I'll otherwise forget it" posts. (After all, that's the justification for this blog.) If you're not interested in fixed-width-font email, move on, nothing to see here.
Why fixed font email? Well, I belong to a baseball analysis email list, and sometimes we want to exchange tabular data. It's always easier to read if you know the width of the tabs. And I grew up with mail read from the command line, or in Emacs.
Besides, HTML email is annoying. . Especially when the font is nearly the same color as the background.
So how to set this up in Thunderbird? Well, Mozilla gives you the basic directions. The only thing you have to remember is that in Linux the Account Settings option is under Edit (Why? Who Knows?) instead of under Tools. So the proper string in Edit > Account Settings > Composition & Addressing > Click "Compose in HTML" Off.
I'll ignore the rest of the article because I occasionally want to forward emails as attachments or otherwise add MIME features. I just want my regular emails to be fixed in place.