root.css

/*
 * F i r e f o x は い い ぞ
 */

:root {
  -webkit-user-select: none;
  -ms-user-select: none;
  -o-user-select: none;
  user-select: none;
}

:root::before {
  content: 'You have to use Gecko or Servo implemented browser. (e.g. Firefox)';
  position: fixed;
}

*:not(:root) {
  opacity: 0;
}

@-moz-document regexp('.*') {
  :root {
    -moz-user-select: auto;
    user-select: auto;
  }

  :root::before {
    content: '';
  }

  *:not(:root) {
    opacity: 1;
  }
}