Well, we can set some options for email composing, but we are still not able to compose in plain text by default. At least not using the options given by the GUI.
I have published once a tip for "how to compose emails in Plain Text instead of HTML". Here it is again, more precise.
If you want thunderbird to compose emails in plain-text format by default, you can edit the file mailnews.js. To find this file, go the directory /Applications/, right-click (or control-click or long-click) the application thunderbird and choose "Display content", then go to Contents, MacOS, defaults, pref. Drag the file mailnews.js to the application "SimpleText".
Find the line
pref("mail.identity.default.compose_html", true);
Change it to
pref("mail.identity.default.compose_html", false);
Restart thunderbird.