Hi KyLane,
Firefox on Mac won't load the video because it doesn't support mp4 video files - only ogv and webm formats. Firefox on Windows does support mp4 video.
Edge Animate CC 2014 displays a warning at the bottom of the Stage panel if your video element doesn't have both an mp4 and ogv/webm source file, advising mp4 and at least one of the other formats (ogv or webm) for widest browser compatibility.
hth,
Joe