The goal is to create an infinitely scrolling, two-layer parallax, patterned background. The effect works perfectly in Mac Chrome 35.0.1916.153, Mac Safari 7.0.4, iOS Chrome 35.0.1916.41, and iOS Safari 7.1.2, but it stutters in Mac Firefox 30.0 (haven't tested on iOS FF). My code is visible at http://jsbin.com/cifig/2 and is basically modeled after http://davidwalsh.name/multiple-background-css-animations (demo http://davidwalsh.name/demo/multiple-background-css-animations.php). If I make either one of the following changes, the stutter in FF resolves: • Limit `background-repeat` to `repeat-x` or to `repeat-y`. • Remove one of the two animated backgrounds. So it seems FF can't handle the continuous animation of two backgrounds that each have background-repeat, whereas Chrome and Safari can. Any tips? Thanks!