Default value of media.cache_size (0) causes some media to load extremely slowly or become unplayable
It seems this value was defaulted to 0 after this ticket: https://trac.torproject.org/projects/tor/ticket/10237
The issue this causes can be seen on this page: https://livestreamfails.com/post/28515
If you open this link in a default configured Tor Browser (I'm using v8.0.4), and open the network inspector (CTRL+SHIFT+E), you will see the media file is constantly being re-requested, downloading the file a few hundred kilobytes at a time using range requests.
This causes the video to load extremely slowly. If the HTTP server does not support range requests, the video will seemingly become unplayable in the native player.
This kind of behavior could also be seen as abusive or mistaken as DoS related traffic by website operators, since it is not typically how browsers download media.
Setting media.cache_size to the default Firefox value of 512000 fixes this, it may also work with a lower cache size, but I've only tried the default Firefox value. I suppose it also may bring back the original issue of the media cache being stored on disk, that ticket is from 5 years ago though, so I'm sure if that situation has changed.
This bug only seems to affect certain media files, I'm not exactly sure what the other factors are in triggering the behavoir.
This bug also exists in the latest stable version of Firefox if you set the media.cache_size to 0, I suppose it's rarely encountered though since the default is much higher.