Every time I go fullscreen in a video like on Facebook, YouTube etc. a message appears at the top saying '(website) is now full screen' with a derp button to 'exit full screen (esc)'. Can I get rid of this incredibly annoying and pointless notification that blocks the view of a video for a good few seconds?
It's browser security thing to stop less scrupulous sites taking over your screen rather than a flash setting, though there is a way round it in Chrome by, essentially, white-listing everything... Go to - Code: chrome://settings/contentExceptions#fullscreen and 'Allow' Code: *://*/* and it should stop asking you... though you still get a brief 'Press [Esc] to exit full screen' popup, bit it's far less intrusive.
With the proviso that I haven't actually tested it, as with most things in Firefox, there seems to be an addon for that: https://addons.mozilla.org/en-gb/firefox/addon/disable-html5-fullscreen-alert/