Like the other something is broke or out of date you tube runs just fine for me and my kids all using Firefox and believe me they watch a lot of You tube. If I had to guess your player is old and needs an update. The only site that does not present right is our troops website on club spaces and that is only the pictures not being in the right places.
I will give it an update
There are several sites I use that didn’t work at all with Firefox until I started running the Internet Explorer emulator plug in. They run faster now than they did with IE.