After all these years, I finally found that I was still experiencing this bug (see my comment 88) because of the addon Enigmail, that automatically sets "mailnews.send_plaintext_flowed" to "false".
After all these years, I finally found that I was still experiencing this bug (see my comment 88) because of the addon Enigmail, that automatically sets "mailnews. send_plaintext_ flowed" to "false".