@-moz-document domain(youtube.com) {
div#theater-background.player-height {
    display: none !important;
}
.watch-stage-mode .player-height {
    height: 55.75vw !important;
}
.watch-stage-mode .player-width {
    width: 100% !important;
    left: 0 !important;
}
.watch-stage-mode .player-width {
    margin-left: 0 !important;
}

/* Took < 2 mins as YouTube has finally simplified their player design to be contained */
}