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Firefox never actually fullscreens
I don't know who at firefox doesn't understand the point of fullscreen, but the address bar continues to pop up when the mouse reaches the top of the screen and there is no setting to turn this off. It is a comically incompetent choice. No, the chrome folder or extensions don't work. Might just switch back to chrome as this is a pattern with firefox's choices.
Wšykne wótegrona (1)
You can use code in userChrome.css to hide the fullscr-toggler and prevent showing the toolbars when you move the mouse pointer to the top of the window.
- /questions/1412553 full screen shows the top bar
Add CSS code to the userChrome.css file in the chrome folder in the Firefox profile folder.
*|div#fullscr-toggler { display: none !important; }