Because I keep forgetting which options to set to force Thunderbird to send and display line breaks instead of flowed plain text emails that may or may not come out right at the other end (usually those emails exhibit unsightly long lines which among other things might make quoting from some MUAs a PITA), here’s a reminder mostly to myself.

Set

• mailnews.display.disable_format_flowed_support to true and
• mailnews.send_plaintext_flowed to false.