ForceFull автор Steven Don

An extremely simple extension which maximises the Firefox window on program startup and switches to fullscreen mode. To exit full-screen mode simply press F11 or forgo switching to fullscreen mode by disabling or removing the extension.

Please note that this works only on program startup, not with every new window opened. Only the primary window will be switched to fullscreen mode, which may be an issue if you try to run multiple screens.