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TL;DR
Go to about:config, look for these settings and change their values.
- To remove the fullscreen fading:
full-screen-api.transition-duration.enter=> 0 0full-screen-api.transition-duration.leave=> 0 0full-screen-api.transition.timeout=> 0
- To remove the fullscreen warning:
full-screen-api.warning.timeout=> 0
Did you ever notice how long does a video go fullscreen on Firefox? I mean, you double click it, the screen goes black, and a second-ish later the video actually appears. I was so intrigued about this (and annoyed at the same time) and began my research about this. The first thing anyone (and me included) would do is to try to google the problem, but this time I took courage and typed on my url bar about:config and then started to try randomly related words like: screen, transition, fullscreen, etc. However, after some tries didn't catched my eye any of the options, anyways I finally achieved what I actually thought was unconfigurable.
So, the actual solution is a mix of some options to reach the desirable transition (the chrome transition, like it's instant). So those options and its values are:
full-screen-api.transition-duration.enter=> 0 0full-screen-api.transition-duration.leave=> 0 0full-screen-api.transition.timeout=> 0
With these set of values the transition feels instant.
Now, on the side of the warning that slides down from the top when you enter on full screen.
If you want to get rid of that is quite easy too. Searching full-screen on about:config is going to give us some wonderful settings to be change. However you are going to focus on the one that says:
full-screen-api.warning.timeoutand this should be set to 0.
By this way the timeout is nonexistent so that means it's never going to show up.