Fullscreen mode (F11) is not supposed to show the Windows taskbar. Some users refer to maximized windows as "fullscreen" which can cause confusion. If you can get the taskbar to appear in actual fullscreen mode and not just a maximized window, that would be considered a bug. A custom Windows theme, registry modification or third-party software can result in non-standard taskbar behavior.

Sorry if I can't tell the difference between fullscreen and maximized.

One of the recent updates to Firefox now has the Win10 taskbar unavailable while Firefox is MAXIMIZED. It should appear when I mouse to the bottom of the screen. Taskbar is in AUTOHIDE mode. I see other complaints about this opened and closed for up to 6 years. No actual answers, just workarounds. Hitting F11 twice solves it temporarily, but it comes back. No, it's not Windows. No other software on my laptop does this. Yes, I keep Firefox updated.

Any progress? THX

This can be caused by graphics overlays like Nvidia's FPS counter. Other third-party software like Stardock Software ObjectDock has also been reported. Try to disable as much background software as possible until it starts working.

Go to about:third-party in the address bar and try disabling third-party modules.