It's the sort of thing that I seem to end up changing about once a year or so when I decide that I prefer a different browser layout.
Currently in fullscreen Firefox (fullscreen in display only, not functionality), I have an autohiding vertical toolbar with autohiding tabbar at the bottom, and that is combined with an autohiding taskbar at the bottom of the desktop, but a few months ago I did have the taskbar at the top of the screen.