At a guess, you have your mozilla
Edit>Preferences>Mail & Newsgroups>Send Format "Convert the message to plain text" option set, and you left your message composer in the default "Variable Width", a condition to which it will stubbornly return itself at every opportunity. ASCII art can be done as long as you keep an eye on the font at all times. I'm using a nightly build though, so YMMV.
Olivier Rossel wrote:
Well, for Ascii art, Vim is probably better :-)