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

@-moz-document url-prefix(https://mail.google.com), 
               url-prefix(http://mail.google.com)
{

  *[title*="Spam ("] {
    visibility: hidden;
  }

  *[title*="Spam ("]::before {
    content: "Spam";
    visibility: visible;
    font-weight: normal;
    text-decoration: underline;
  }

}