PS: I am on 38.2.0 under Windows 10

this forwarded text was originally one single line and send as plain text between my accounts

Just moved to Thunderbird. When I forward an email, the forwarded text does not word-wrap. The new email text wraps fine. This behaviour seems to be general, and I can find no reference to it in "Help". Is it a bug?

It doesn't show here but when I entered the single line and send it to myself as plain text it looked wrapped. But wasn't. The received mail did not contain CR LF thou it viewed as wrapped I forwarded this (still as plain text) and both the forwarded send-copy and the second received mail had CR LF

So my experience is the opposite it does wrap and it adds CR LF when forwarded even if there were non to begin with

Here might be your solution http://kb.mozillazine.org/Plain_text_e-mail_%28Thunderbird%29#Flowed_format

Belated Thanks, Gnospen. I suspect the answer is in that link you sent, which I will dig through! I've not in fact noticed the problem much since I first posted.