Mozilla tweaked Firefox's full screen behavior in recent Nightly versions of the browser (that is version 43) to make it less annoying to users.
Some sites allow you to run applications or services in fullscreen. Many video streaming services such as YouTube or Twitch offer full screen options to stream videos in full screen.
This fullscreen option should not be confused with running the browser itself in fullscreen mode. The latter is toggled with a tap on the F11 key on the keyboard.
Firefox users who have made use of that option until now had to permit full screen mode explicitly each time it was activated on a site supporting it. This meant that the video would not start streaming before explicit permission was given with a click on the allow button.
Firefox offered two options until now to get around this. Site permissions allow you to set specific permissions for select sites, and one of the permissions is full screen mode. If you set it to allow, the prompt to allow it on activation is no longer displayed for that site.
The same option is displayed on the prompt when you activate the fullscreen option on a site. All you had to do was check the "remember decision" button to save the permission change.
The second option was to set the preference full-screen-api.approval-required to false. This disabled the approval requirement for all sites and services.
The change removes the explicit action from the process. A prompt is still displayed when fullscreen mode is activated on a site like YouTube, but it won't require explicit permission to run anymore.
Firefox displays a short prompt revealing that the site is running in fullscreen mode and that the mode can be exited again with a tap on the Esc key on the keyboard.
The prompt disappears after a couple of seconds and won't come up again until you exit fullscreen mode and re-enter it at a later point in time.
The feature landed in Firefox 43 Nightly and the target milestone for it is Firefox 43. That's three release cycles for stable versions of the browser, two for beta and one for Developer editions if things go as planned.
Please note that Mozilla removed the fullscreen preference on Firefox's about:config page as a consequence as it is no longer needed.
Update: Apparently, there is still a preference available to define fullscreen behavior and turn it completely off. This works in Firefox 43 or later only. What you need to do is set the preference full-screen-api.warning.timeout to a value of 0 on about:config.
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