It seems like there is a loading error that may be coming from your server. We could load the video locally on FF.
This is the error we found:
“Load of media resource https://brain.3ww.nl/wp-content/uploads/2014/11/Marion-changescape.mp4?_=1 failed.”
I think you might want to check in with your host on this.
Also, would you use the WP video shortcode and see if you get the same error? https://codex.wordpress.org/Video_Shortcode