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

@-moz-document domain("mail.google.com") {
 a[title^="Spam ("] { 
  visibility: hidden !important; 
 }
 
 a[title^="Spam ("]:before { 
  content: "Spam"; 
  visibility: visible; 
  font-weight: normal; 
  text-decoration: underline;
 }
}