press F11 set firefox into fullscreen mode but taskbar still visible

Hi enderman_how, Firefox's F11 full-screen mode is intended to cover the whole screen, including the desktop panel/taskbar. Mozilla's documentation confirms that F11 switches Firefox between normal and full-screen mode. Since you're using Linux, could you tell us which Linux distribution and desktop environment you're using (for example, Ubuntu with GNOME, Kubuntu with KDE, Linux Mint with Cinnamon, etc.)? If the taskbar remains visible specifically when Firefox is in F11 full-screen mode, while other applications can hide it normally, that may be related to how your desktop environment handles fullscreen windows rather than a Firefox setting.