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

@-moz-document  url(about:blank) {
  html:before {
    display: block;
    font-family: arial;
    font-weight: bold;
    font-size: 1.5em;
    padding: .6em 0em .2em 1em;
    background-color: buttonface;
    border-bottom: 1px solid buttonshadow;
    color: buttontext;
    content: 'about:blank'; /* Change if you want */
  }
}