You can modify some settings to shorten the notification or hide it completely. But I'm not sure that will help with the issue of exiting fullscreen. You can look at settings in that same area to see whether any of them help.

(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 or accepting the risk.

(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 edit the value to 0 (that's a zero)

(4) Double-click the full-screen-api.warning.timeout preference and edit the value to 200 (that's 2 tenths of a second, just a very brief blip)

(5) There are three preferences starting with full-screen-api.transition. that could affect the other issue, but I've never experimented with them myself.

If you change them in a way that is worse, you can right-click > Reset to restore their default values.

Let us know what you come up with.

the Full-scree-api.transition-enter and -leave should be set to 0 0 (2 zeros) and that pretty much almost remove the animation. Thank you for your help with the fullscreen message :D

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To avoid confusion, that would be these two prefs:

• full-screen-api.transition-duration.enter: "0 0"
• full-screen-api.transition-duration.leave: "0 0"