I would just turn off the new feature.

The paragraph feature is already turned off and any composition is in single line format. However when reply to a message from someone else that is originally in single line format, the quoted text appears in double lien format.

Is that "someone else" any one person in particular, or everybody?

Can you provide a screenshot to show us what you're seeing?

https://support.mozilla.org/en-US/kb/how-do-i-create-screenshot-my-problem

Original message https://support.cdn.mozilla.net/media/uploads/images/2016-05-19-05-36-18-f97c5d.png

My reply https://support.cdn.mozilla.net/media/uploads/images/2016-05-19-05-38-32-b8a9f7.png

Couple of things.

Try safe mode. Hold the shift key while starting Thunderbird and continue with add-ons disabled when prompted. Any change?

Zeno asked if it was only one person. I will be more pointed. Is it only mail from John and Pam?

I was looking for a simple answer for the double line spacing when I reply with a quote. There doesn't seem to be a simple answer. I'll just live with the double line spacing. I don't have time to continue with this request and will just mark the request solved.