For many years I've used Thunderbird with Alexandre Feblot's external editor plugin to allow me to edit mail with emacs. Unfortunately it seems long unmaintained and stopped working on a recent upgrade to Thunderbird 54 when some deprecated interfaces were removed. Brian M. Carlson seemed to have another version which also seemed to fail with latest Thunderbird.
I have used my meagre Mozilla plugin skills to make an update at https://github.com/ianw/extedit/releases. Here you can download an xpi that passes the rigorous test-suite of ... works for me.