That would explain blocking of Firefox, but they're blocking Safari and WebKit nightly and... everything else that doesn't have "Chrome" in User-Agent string: http://yfrog.com/z/kh1rylsqj
this.src = src.replace(/(\.webm+)$/, (navigator.userAgent.indexOf('Chrome/') == -1 && (navigator.userAgent.indexOf('MSIE') != -1 || navigator.userAgent.indexOf('Safari/') != -1)) ? '.mp4' : '.webm');
but looks like they didn't upload any mp4 files :( sad