It used to be possible to use an external editor, such as Emacs, to edit Thunderbird mail messages, but now I haven't found a way to do it. The solution to this question posted 7 years ago doesn't work on the more modern versions of Thunderbird (I'm running v. 78.13.0). I also tried using the code offered here, but unsuccessfully.
Does anyone knows if there's another solution for editing Thunderbird messages in Emacs or if someone's working on updating previous hacks?