Have you tried the old way with the pointer at the upper screen level to show the adress bar and then right click to get menu to set it always show in full screen.
What does going to the adress about:config show?
Look for the setting below:
browser. fullscreen. autohide Boolean Hides toolbar and tab strip when entering Full Screen.
True (default): Shows toolbar and tab strip.
False: Hides toolbar and tab strip
Google "firefox about:config" as the link does not compute in forum.