When it says "edit your thunderbird config settings" it means use the edit->preferences menu, go to the "advanced" tab, and then use the "config editor" button near the bottom of that page. (Smell the usability! Kmail had an options->wordwrap in a given composer window. This has a four step process to access a windows-style registry editor.)
Now, when you compose messages, you have to hit ctrl-R to manually wordwrap the thing. Instead of making an exception for posting patches, there's an extra manual step in composing normal email.