thunderbird.md

Manual advanced configuration (about:config)

user_pref("general.useragent.override", "");
user_pref("mailnews.headers.showUserAgent", true);
user_pref("mailnews.wraplength", 0);

If support for the Markdown Here extension is needed, the first preference needs to be like this:

user_pref("general.useragent.override", "Thunderbird");