gecko-dev/layout/reftests/bugs/664127-1.css

/* This external stylesheet is needed to be able to use a cached image
   (solidblue.png) through a relative path */
tree {
  height: 100px; -moz-appearance: none; border: none; background-color: transparent;
}
treecol, treecolpicker {
  visibility: hidden;
}
treechildren::-moz-tree-image {
  list-style-image: url('solidblue.png');
}