First, if you have any addons, turn them all off and retry. If that doesn't work, this next approach may be worth a try... go to settings>general and click on the config editor and look for mail.compose.default_to_paragraph and change the value there.

Thank you david. The very first thing I did is to de-activate or uninstall add-on. No effect at all. The boolean parameter that allows to switch from true to false mail.compose.default_to_paragraph can be changed but automatically reverts to its original and apparently default value ("True") after you quit Thunderbird. And this behavior is not the one expected I suppose. So it looks that old bug (I have been facing this issue for years) persists, for unknown reason. Regards,