Hmm, seems to be a weird bug in older Webkit’s. If the menu button link has display: inline set and no real text content (apart from that added via :before), it isn’t clickable. So you have 2 options:
I) Either set the following in the CSS:
http://e-ms.co/dev/mobilechildtheme/wp-content/themes/mobilefirst/style.css
#menu-button {
display:inline-block;
...
}
II) Or add some real text inside the link here (and maybe add just the icon via :before):
jQuery("nav .genesis-nav-menu").before('<div id="menu-button-container"><a id="menu-button" class="null">MENU</a></div>');
