MediaWiki:Common.js

var isMainPage = (document.title.substr(0, document.title.lastIndexOf(" - ")) == "Main Page");
var isDiff = (document.location.search &&
                 (document.location.search.indexOf("diff=") != -1 ||
                     document.location.search.indexOf("oldid=") != -1
                 )
             );
if (isMainPage && !isDiff) {
    document.write('<style type="text/css">/*<![CDATA[*/ h1.firstHeading { display: none !important; } /*]]>*/</style>');
}