mobile-config-firefox/src/userChrome/browser.css

/* Copyright 2021 plata
 * SPDX-License-Identifier: GPL-3.0-or-later */

/* Move navigation bar to bottom */
@media (max-width: 700px) {
    #browser {
        -moz-box-ordinal-group: 0 !important;
    }

    /* Hide navigation bar in kiosk mode (to prevent bug #29). We can assume FF
     * is in kiosk mode when fullscreen and max-width conditions are met,
     * because at this max-width the fullscreen button is hidden
     * (see appMenu.css). */
    #nav-bar[inFullscreen],
    #TabsToolbar[inFullscreen] {
        display: none;
    }
}