This post is more than 2 years old.
For a long time I've been bugged by something in Thunderbird. I'd hit reply to someone, and the previous email, when quoted, would turned into long lines. In other words - it wouldn't wrap.
For the heck of it - I went into the Edit menu when on an email like this, and noticed "Rewrap" as an option. It did exactly that - rewrapped. Why didn't I see this months ago?