Firefox video not playing issue on CentOS/ Linux/ Windows :
PLayback not supported by this device
If you’re facing issue with playing YT (YouTube) videos on your Firefox browser install this add-on HTML5 Loop from this link >> https://addons.mozilla.org/en-US/firefox/addon/html5-loop/
Click on +Add to Firefox and install it as shown in below snapshots.
Once you install it you’ll see it in My Add-ons:
It perfectly works. Try it.