This was not working for me in Firefox.
We added a conditional based on whether the code places the overflow at the html level. See Animate scrollTop not working in firefox.
var $header = $("#header #menu-wrap-left"),
$clone = $header.before($header.clone().addClass("clone"));
$(window).on("scroll", function() {
var fromTop = Array();
fromTop["body"] = $("body").scrollTop();
fromTop["html"] = $("body,html").scrollTop();
if (fromTop["body"])
$('body').toggleClass("down", (fromTop["body"] > 650));
if (fromTop["html"])
$('body,html').toggleClass("down", (fromTop["html"] > 650));
});