C, right-click + Copy or whatever you're normally doing. The text is copied as plain text, without any formatting.
Works on the body (not the subject, sender etc.) of emails that are being read in the main Thunderbird window, in a separate window, or in a tab. Does not work in a "New email" composition window.
Many predecessors exist, most with a lot more functions and options, but they don't work on Thunderbird from version 78, when the extension system changed to MailExtensions: