CSS - :-moz-window-inactive

This example alters the appearance of a box's background depending on whether its window is active or not.

<style type="text/css">
#mybox {
  background: linear-gradient(to bottom, blue, cyan);
}

#mybox:-moz-window-inactive {
  background: cyan;
}
</style>

<div id="mybox" style="width:200px; height:200px;">
  <p>This is a box!</p>
</div>

You can view this as a live sample here.