It isn't working in FF, because Firefox does not support transitioning a background image, only background-color.
If you want to transition a background image, use two separate <div>
:
.fade div {
-o-transition: 0.3s;
-moz-transition: 0.3s;
-khtml-transition: 0.3s;
-webkit-transition: 0.3s;
-ms-transition: 0.3s;
transition: 0.3s;
width:128px;
height:128px;
position:absolute;
top:0;
left:0;
}
.fade{
position:relative;
width:128px;
height:128px;
}
.backone{
z-index:1;
background:url('http://nuclearpixel.com/content/icons/2010-02-09_stellar_icons_from_space_from_2005/earth_128.png');
}
.backtwo{
background:url('http://upload.wikimedia.org/wikipedia/commons/b/b2/Crystal_128_babelfish.png');
opacity:0;
z-index:5;
}
.fade:hover .backtwo{
opacity:1;
}
.fade:hover .backone{
opacity:0;
}
<div class="fade">
<div class="backone"></div>
<div class="backtwo"></div>
</div>
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