Thunderbird: line wrap in plain text mails doesn't work

There are other advanced settings you need to change besides ''mailnews.wraplength''. Namely, they are: '''''mailnews.send_plaintext_flowed''''' - set this to false so that the plain text messages '''you send''' do not support the flowed format. This means, the recipient/viewer of the message will see the message wrapped at the 75th character (or whichever wrap length you set). The message will not support being reflowed to occupy the viewer's window. '''''mailnews.display.disable_format_flowed_support''''' - set this to true to disable support for the flowed format when '''viewing''' a plain text message, even if that message was sent with the flowed format enabled.