Pi77Bull said

I changed "full-screen-api.warning.timeout" to "0" in "about:config".

Hi   ! Have you also set the value of  'full-screen-api.approval-required'  
to   'false'   ?

I didn't even had this setting. I added it and set it to false but it didn't change anything.

Bummer   !
Would you consider this add-on;   see if that'll do the trick:
https://addons.mozilla.org/en-US/firefox/addon/disable-html5-fullscreen-alert/

That's an interesting problem, when the cursor is not moving back onto the content but is perceived as continuing to be at the top of the screen.

First, to allow the warning to animate off the screen, try this combination of timings:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful.

(2) In the search box above the list, type or paste full-s and pause while the list is filtered

(3) Double-click the full-screen-api.warning.delay preference and enter 0 (that's a zero)

(4) Double-click the full-screen-api.warning.timeout preference and enter 50 (you can play with this as needed)

What I see when I hover at the top is a blip of the bottom of the warning before it slides back up out of sight. How about you?

Second, to make the warning invisible regardless of how it animates on/off the screen, you could use a custom style rule such as the following:

div#fullscreen-warning[class="pointerlockfswarning"]{
  opacity: 0 !important;
}

Note: Opacity determines how transparent an element is, but the actual value is the opposite. 0 opacity is completely transparent, while 1.0 opacity is "100%" solid. 0.5 opacity is faded halfway. Using a value like 0.2 provides a subtle message.

You can apply custom style rules to Firefox's user interface using either the Stylish extension or a userChrome.css file. But I'll spare you the details because I think the short blip probably will be tolerable to you.

@Happy112 So the Addon doesn't work. It does exactly the same what I have done to the settings of Firefox.

@jscher2000 I tried doing everthing you did but instead of seeing this short blip you were talking about, the fullscreen alert comes up. Also, I tried using the code you gave me with the extension Stylish (I had already installed) but it didn't work either (I probably made mistakes though as I don't really know how to use it :D )

BUT I found a solution to my problem:

I just had to set "full-screen-api.warning.delay" to any negative integer like -1 instead of 0. This did the trick.

Anyways, thanks to everyone who tried to help me!

Hi Pi77Bull, on this one point --

Also, I tried using the code you gave me with the extension Stylish (I had already installed) but it didn't work either

-- the trick is to create a new blank style and paste in just the rule, without any namespace.

Pi77Bull said

I just had to set "full-screen-api.warning.delay" to any negative integer like -1 instead of 0. This did the trick.

This seems to work for me while Firefox remains open, but the pref resets to 0 next time I start the browser. This would be great if I could get this to stick indefinitely.

Hi PAGulley, I haven't tested that myself. The following article describes some ways to investigate causes for losing settings between sessions:

How to fix preferences that won't save

I know this is an old issue, outta site, outta mind, but would it be possible to also check if you change security.sandbox.content.level = 0