title: backgroundPositionX not working on Firefox
body: <p>I have play with youtube's sprite animation but there is a problem. <code>backgroundPositionX</code> won't work under Firefox (but works on Chrome and IE8)...
  This is the code: <a href="https://jsfiddle.net/74RZb/" rel="nofollow noreferrer">https://jsfiddle.net/74RZb/</a>  </p>
  
  <p>Extra info: the problem is that under firefox It doesn't change the background position (won't play the animation)... there are no errors, just doesn't change the background position.</p>