CSS - using ':before' to place an image not appearing in Firefox

CSS - using ':before' to place an image not appearing in Firefox

I'm trying to position a temporary 'beta' ribbon image positioned in the
top left of a wordpress based site.
The following code functions as intended in Chrome and IE. In Firefox,
nothing is appearing:
#page-wrapper:before
{
width: 50px;
content: " ";
background: url(wp-content/uploads/2013/09/betaribbon2.png) no-repeat;
position: absolute;
height: 50px;
pointer-events: none;
}
Can anyone shed light on what I'm doing wrong?
Thanks,
LB