Perhaps you are one of those that still liking to read the emails like in the old times, i.e. plain text and 72 characters wide.
For Mozilla’s lovers, it is quite simple to enable such thing, just go to:
Preferences -> General -> Config Editor, and configure the following values:
mailnews.display.disable_format_flowed_support boolean true
mailnews.send_plaintext_flowed boolean true
And of course make sure that mailnews.wraplenght is 72 (default)
Voilá, now you have your email client configured according with the old RFC-196 (in terms of characters wide)
Just don’t forget to have enabled in send options “Convert the message to plain text”.