http://meta.stackoverflow.com/a/142217/169101

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

@-moz-document domain('stackoverflow.com'),
               domain('stackexchange.com'),
               domain('superuser.com'),
               domain('serverfault.com'),
               domain('stackapps.com'),
               domain('askubuntu.com'),
               domain('mathoverflow.com'),
               domain('answers.onstartups.com') {
  /* old StackExchange top bar, e.g. on stackexchange.com */
  #portalLink {
      position: fixed;
    /*left: 22px;/* Uncomment to stick to the left */
      background-image: -moz-element(#custom-header) !important;
      background-size: 1px !important;
      background-repeat: repeat !important;
      padding: 8px 6px 6px 16px !important;
      margin: -2px 0 0 -22px !important;
      border-radius: 5px;
  }
  body {
      padding-top: 34px; /* reserve height of .topbar */
  }
  .topbar {
      position: fixed !important;
      top: 0;
  }
}
@-moz-document url-prefix('https://stackexchange.com/'),
               url-prefix('http://stackexchange.com/') {
    #portalLink {
        background-image: -moz-element(#topBar) !important;
    }
}