Disabling line wrapping in text emails I send is one of the first settings I change on every installation on a new machine. This way emails sent to mailing lists are much easier to read, especialy on web archives. But I always forget which key need to be modified, so here's a little reminder for me, and anyone who doesn't like to read huge lines of non-wraped text. You need to set to false the key mailnews.send_plaintext_flowed.

More details here: http://arapulido.com/2009/12/01/enabling-line-wrapping-in-thunderbird/