For future reference, add http:// in front of your URL or use the link tags so that it is clickable (much easier to access).
background-size is a newer property that is not supported in older versions of Firefox. In other words, you can't have a variable sized background image. However, it is possible to simulate a background image with a fixed, z-indexed image. Something like the following:
<div id="my-bkg-img"><img src="http://2.bp.blogspot.com/-03LnaUK9Ud8/TcZC3knAoZI/AAAAAAAAAxc/QRm7Pp5cx-o/s400/Penguins.jpg" id="my-bkg-img" /></div>