[**Stylish**][1] is your friend. I check every day or so to see if it's still there. When it goes I'll turn the style off. 

    @namespace url(http://www.w3.org/1999/xhtml);
    
    @-moz-document domain("serverfault.com") {
    #system-message { display : none ;}
    }


  [1]: https://addons.mozilla.org/en-US/firefox/addon/2108/