Detect hi-dpi screens using Javascript with a fallback to a matchMedia query on Firefox

window.devicePixelRatio Detection

// Ala Thomas Fuchs : retinafy
function retinaDevice() {
    return (('devicePixelRatio' in window && devicePixelRatio > 1) || 
            ('matchMedia' in window && !matchMedia("(-moz-device-pixel-ratio:1.0)").matches))
}