Hello coremedia, I assume you are talking about the toolbar on your Firefox browser not being hidden when in Fullscreen Mode? If so, when you enter Fullscreen Mode (by tapping F11), right-click your toolbar and check the "Hide Toolbars" option. Next time when your cursor is not hovering over the area where the toolbar normally is, the toolbar should automatically be hidden. Refer to the attached screenshot if you are unsure what to right-click. Hopefully this answer helps, if it did, kindly mark this answer as the solution(solved)!