gistfile1.js

jQuery(document).ready(function() {

  /*
    Snippet from http://stackoverflow.com/a/5918791
  */

  navigator.sayswho = (function(){
		var N= navigator.appName, ua= navigator.userAgent, tem;
		var M= ua.match(/(opera|chrome|safari|firefox|msie)\/?\s*(\.?\d+(\.\d+)*)/i);
		if(M && (tem= ua.match(/version\/([\.\d]+)/i))!= null) M[2]= tem[1];
		M= M? [M[1], M[2]]: [N, navigator.appVersion,'-?'];
		return M;
	})();

	var headerOffset = jQuery(".main-header").height(),
		blockMenu = jQuery(".block-menu"),
		nameNavigator = navigator.sayswho[0];

	if (nameNavigator == "Chrome") {
		var bodyelem = jQuery("body");
	}

	else if (nameNavigator == "Firefox") {
		var bodyelem = jQuery("body, html");
	}

	else if (nameNavigator == "Safari") {
		var bodyelem = jQuery(document);
	}

	else if (nameNavigator == "MSIE") {
		var bodyelem = jQuery(window);
	}

	jQuery(window).scroll(function () {
		if (bodyelem.scrollTop() >= headerOffset) {
			blockMenu.addClass("is-mini").slideDown("slow");
		}
		else {
			blockMenu.removeClass("is-mini");
		}
	});
});

Forked it : https://gist.github.com/mlbli/5520684