Webkit+Moz detect device pixel ratio

gistfile1.js

var ratio = window.devicePixelRatio ||
    (function(i){
        var wmq = window.matchMedia;
        while( !window.devicePixelRatio && i > 0 &&
               !wmq("(min--moz-device-pixel-ratio:" + i/10 + ")").matches ){
            i=i-1;
        }
        return i/10;
    })(30);

I just searched google for this code and this appears to be exactly what I was looking for. How reliable is this? I am working on a cross-browser solution to serve retina icons for Google Maps.

Would this work on Firefox too? Obviously I will test this too, just wanted to get your feedback.

if(ratio > 1.5) {
    console.log('serve high res graphics');
}