What is happening when you press F11?

These are some old issues that haven't changed recently:

(1) Firefox won't go completely full screen if the cursor is in the address bar -- you will notice Firefox expand its window to covers the Windows Taskbar, but still showing the toolbars.

If that happens, is the problem, you need to click in the document, then press F11 twice to try again.

(2) If the toolbars don't disappear even with the cursor in the document and the Windows Taskbar covered, try right-clicking the + (new tab) button on the tab bar and look for a check mark before "Hide Toolbars".

If there's no check mark, click "Hide Toolbars" to turn that back on.