For composing messages, Edit/Preferences/Advanced/General/Config. editor, right-click the preference mailnews.wraplength, Modify, and set it to a value of 72 or 80. A value of 0 will wrap to the window width. This applies to plain text messages.

As for received messages, when they don't wrap, it's usually some non-standard formatting by the sender's email program. Sometimes, it helps to read such messages by opening a Forwarding window, then Ctrl+R (Edit/Rewrap).

Thank you sfhowes.

That is the expected behavior. My Thunderbird is not behaving that way.

My mailnews.wraplength is set to 72 which also looks to be the default. It is not wrapping to that length, after a restart of course.

I can (re)produce this behavior by clicking write to begin an new email. Then type. It will go on forever until I press enter.

As for the received emails, you are also correct. Those were test emails and they were form ME! I am the one with the badly formatted messages.

I am using Thunbderbird 60.9.0 on Linux Mint 18.3.

I am at a loss. I will investigate more. Anyone have any idea what I should do next?

Ok problem solved. The original text was cut and pasted form elsewhere. It was retaining formatting. The solution is to paste and then Ctrl+R .

Thanks again.

When copying from another app, it's best to Edit/Paste Without Formatting (Ctrl+Shift+V).