Make fullscreen transition not push the website content

Current behaviour in fullscreen mode: When I move my mouse to the top of the screen The tabs appear and the whole website content gets shifted down. It's ugly and distracting. Desired behaviour: The tabs appear as an overlay and part of the website content is hidden underneath them. No transitions.

What css js should I modify to achive this, any help? This prevents me from using firefox in fullscreen mode and saving space.