Mozilla improves video full Screen viewing experience in Nightly by removing the annoying message that seeks your permission. At present in Firefox 40, whenever you try to watch a video on YouTube in full screen, you’ll be shown a popup with a message, until you click on ‘Allow’, you won’t be allowed to pause or interact with the video in full screen.
That’s annoying, but remember that message showing is there in Chrome and other browsers also as a security measure to prevent phishing attacks. You can remove that message by disabling full-screen-api.approval-required preference.
Firefox 43 lets you Watch YouTube Videos in Full Screen without asking for permission
Without compromising the security, Mozilla has tweaked the notification that appears in full screen, here it doesn’t ask for your permission, but a message will still be shown to the user like ‘youtube.com is now full screen Exit Full Screen [Esc] ‘, the message automatically goes away after 3 seconds without requiring the user interaction. Great! Isn’t it.
Mozilla has made a video on this, watch it on YouTube. You can experience this in latest Nightly version.
Note: As a part of this change, fullscreen api approval required preference has been removed from about:config in Nightly.