Comment 7 for bug 1647198

I am able to play the screencast in my Firefox installation as well as the default Ubuntu player. So I think that it's probably an issue with your environment instead. Have you tried downloading it and playing it in a native player? MP4 is a very common format so you should be able to find some way of playing. If however you do not manage, what format would you prefer me to upload it in?

Yes, this issue showed up in a few of the previous releases. Though the issue is fairly recent. It just happens more in the later versions.