Mine wraps to fit my window. In the vast majority of cases, it would do the same to fit to my correspondents' display. In both of these situations it would usually re-flow to fit if the display window width was changed. I wonder what you have done to break this behaviour?

I suggest you go to Tools|Options|Advanced|General→Config Editor and review these settings:

plain_text.wrap_long_lines mailnews.wraplength mail.wrap_long_lines mailnews.send_plaintext_flowed mailnews.display.disable_format_flowed_support

Hmm, are you using plain text and have you deliberately and specifically set the line length to the current setting? If so, the answer is in your own hands.

I got the message that these were factory settings, and to be very careful. I have never changed any of these settings so I don't think that changing any of these settings would restore my e-mail writings to what existed before yesterday.

"…existed before yesterday…" Eh? So this has just happened, spontaneously? Your question makes it sound like a long-standing issue.

All messages you send? All your correspondents? Or just one message, or just one correspondent, or just today?

The problem started yesterday. I have tried sending several test messages, and the problem has been with all of them. I did not see the note in the previous message about setting the line length. How do I set the line length? Thanks

It depends on how you are composing. In HTML mail you typically don't set a line length, but in plain text you can.

See this article for more info about the settings Zenos posted. You don't have to worry about the warning about your warranty, that message isn't meant too seriously.