SeaMonkey Mailnews FAQ
What is Format = Flowed, and how do I de-activate it (classic line break)?
This standard lets you send text with returns only at the ends of paragraphs, which is much more convenient for reading on small or unconventional displays (smartphones, etc.).
The way format=flowed works is by sending your mail wrapped and with normal quote characters (>'s), just like before.
The only difference is that whenever SeaMonkey wraps your text, it adds a single space to the end of the line.
This allows SeaMonkey (or other format flowed mailers) to reconstruct the original text exactly, taking out all the line breaks that were added, and presenting legible mail to the reader, no matter how wide or narrow the window is.
A simple mailer won't know how to do this, of course, but that's ok; the only difference will be a single space added on the end of each wrapped line, which will disturb no one.
Format=Flowed is a display thing, the message is still wrapped as set in the preferences.
For deactivating the display of format=flowed enter about:config in the browser location bar.
Search for the preference mailnews.display.disable_format_flowed_support.
Modify the value to true.
To also deactivate the sending in format=flowed (and therefore get rid of format=flowed completely) enter about:config in the browser location bar.
Search for the preference mailnews.send_plaintext_flowed.
Modify the value to false.