I have a background image covering the entire width and height of the browser screen. It works in chrome and IE but in Firefox the image is being pushed down. Any ideas what's happening?

```
/* CSS */
<style type="text/css">

#bg {
position:fixed;
top:0;
left:-50%;
width:200%;
height:75%;
}
#bg img {
position:absolute;
top:0;
left:0;
right:0;
bottom:0;
margin:auto;
min-width:50%;
min-height:50%;
}

</style>

/* HTML */
<div id="bg">
<img src="http://image.iloqal.com/lib/fe5c1570746107757c1c/m/1/BMW_528_bkg.png" id="back1" alt="" />
</div>

```