Usefull fallback here: https://developer.mozilla.org/en-US/docs/Web/Events/MozOrientation
function orientationhandler(evt){
// For FF3.6+
if (!evt.gamma && !evt.beta) {
evt.gamma = -(evt.x * (180 / Math.PI));
evt.beta = -(evt.y * (180 / Math.PI));
}
// use evt.gamma, evt.beta, and evt.alpha
// according to dev.w3.org/geo/api/spec-source-orientation
}
window.addEventListener('deviceorientation', orientationhandler, false);
window.addEventListener('MozOrientation', orientationhandler, false);