this add on does what it says, but not what i expected. it puts firefox in fullscreen mode, not the webpage itself

I'm trying to keep Firefox in fullscreen to save my OLED from burn-in so I thought this addon is just what I needed.

It works but flickers crazily every minute even though it is already in full screen. I hope it can be fixed because it is unusable as it is, and I need an addon like this.