@namespace url(http://www.w3.org/1999/xhtml);

@-moz-document domain("mail.google.com") {
  /* for Gmail 1.0 */
  div.msg > div.mb {
    font-family: monospace ;
  }
  /* for Gmail 2.0 */
  div.YrSjGe > div.ArwC7c
  {
    font-family: monospace ;
  }
}