FIX: Email Previewing was not showing in Firefox

app/assets/javascripts/admin/controllers/admin-email-preview-digest.js.es6

@@ -8,10 +8,6 @@ export default Ember.Controller.extend({
   showSendEmailForm: Em.computed.notEmpty('model.html_content'),
   htmlEmpty: Em.computed.empty('model.html_content'),
 
-  iframeSrc: function() {
-    return ('data:text/html;charset=utf-8,' + encodeURI(this.get('model.html_content')));
-  }.property('model.html_content'),
-
   actions: {
     refresh() {
       const model = this.get('model');

app/assets/javascripts/admin/templates/email-preview-digest.hbs

@@ -43,7 +43,7 @@
       {{#if htmlEmpty}}
         <p>{{i18n 'admin.email.no_result'}}</p>
       {{else}}
-        <iframe src="{{iframeSrc}}" />
+        <iframe srcdoc={{model.html_content}} />
       {{/if}}
     {{else}}
       <pre>{{{model.text_content}}}</pre>