This issue is a follow-up of EZP-25918.
EZP-25918 seems to resolve this issue in chrome and in opera, but it still occur on firefox (v49).

As verified previously, use prod environment to reproduce the issue

STEPS TO REPRODUCE

• Open firefox
• Create a media (new contenttype, with a video fieldtype) object and insert one mp4 video (Video1.mp4) - When inserting the video, one can see that the preview is ok and the video plays correctly
• Publish the video. Once again, the preview is ok and the play is correct as well
• Edit the object and replace the video (Video2.mp4). Here we can see that the video name is updated, but the video preview is still from the first video (Video1)
• Publish the video. Once again, the name is updated, but the presented video is the first one, Video1.mp4
• This seems to be a browser cache problem, because if we clear the caches, the video is rightly displayed.