In my case the Tor Browser can't play MP4 files. An example would be this site:
While the WebM file plays fine I get prompted to download the MP4 file because "Tor Browser cannot display this file".
YouTube is working using HTML5.
I noticed that the Tor Browser doesn't have the OpenH264 plugin maybe that is the reason?
For testing purposes I tried installing the OpenH264 plugin or getting it to download by setting "media.gmp-provider.enabled" to true in about:config but it would only say that the plugin will be installed shortly. I don't remember the exact wording sorry.
Does someone else have this problem if it even is one?