Last I checked (which was a while ago), while the UI is in XUL / JavaScript, the bulk of the mail handling (POP/IMAP/SMTP, MIME parsing, etc.) was still in C++ that was extremely complex and hard to port to JavaScript. There was an attempt to rebuild a mail client in JS for FirefoxOS (the mobile phone thing that is now mostly KaiOS), but that's not used in Thunderbird as far as I know.