autohide the panel

[LL-user]

That's correct, as soon as you close the window the "Always on Top" setting is gone.

To actually get the maximized window overlap the panel you also need to activate "Don't reserve space on borders" in your panel setting.

It's all not elegant I know

Another way to get as much screen estate as possible with Firefox is running it in full screen mode. That might be a lot easier than tweaking all the panel, workspace and window settings. Just hit F11 to activate, and F11 again to deactivate