Sorry firefox you don't get fulllscreen

static/css/application.css

@@ -2878,6 +2878,10 @@ section.course-index div#accordion ul.ui-accordion-content li a p {
 section.course-index div#accordion ul.ui-accordion-content li a p.subtitle {
   color: #666; }
 
+@-moz-document url-prefix() {
+  a.add-fullscreen {
+    display: none  !important; } }
+
 section.course-content .dullify, section.course-content div.video-subtitles div.video-wrapper section.video-controls ul.vcr, section.course-content div.video-subtitles div.video-wrapper section.video-controls div.secondary-controls {
   opacity: .4;
   -webkit-transition-property: all;

templates/sass/courseware/_video.scss

+@-moz-document url-prefix() {
+  a.add-fullscreen {
+    display: none !important;
+  }
+}
+
 section.course-content {
   .dullify {
     opacity: .4;
@@ -290,6 +296,7 @@ section.course-content {
             @include transition();
             width: 30px;
 
+
             &:hover {
               color: #fff;
               text-decoration: none;