Ever been annoyed that GMail displays emails in text/plain in a non-proportional font? Me too. Add this to chrome/userContent.css in your Firefox profile, and be happy.

userContent.css

@-moz-document domain(mail.google.com)
{
  .gs .ii,
  textarea.Ak,
  table tbody tbody tr td font,
  div[aria-label="Message Body"],         /* compose area */
  table[aria-role="presentation"] .iA.g6, /* excerpts in conversation view */
  tr.zA.yO span.y2,                       /* excerpts in inbox view */
  .jj                                     /* gmail offline */
  {
    font-family: monospace !important;
    font-size: small !important;
  }
}