Magento - Facebook Like box only shows up in Chrome

Question

Recently I added the 'Facebook Like Plugin' to my website. However, the Facebook plugin only shows up in chrome:

Chrome: facebook iframe in chrome

Safari/IE/Firefox: facebook iframe in ie/firefox/safari

I have searched for ages now, and I already tried changing http:// to https:// and to // only.

What am I doing wrong?

Thanks!

<div class="fb-like" data-href="https://www.facebook.com/*mysite*/?fref=ts" data-width="300" data-layout="standard" data-action="like" data-show-faces="true" data-share="true" margin-bottom="5em;"></div>

(function(d, s, id) {
var js, fjs = d.getElementsByTagName(s)[0];
if (d.getElementById(id)) return;
js = d.createElement(s); js.id = id;
js.src = "//connect.facebook.net/nl_NL/sdk.js#xfbml=1&version=v2.5";
fjs.parentNode.insertBefore(js, fjs);
}(document, 'script', 'facebook-jssdk'));