@namespace url(http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul);
.tabbrowser-tabs .tab-icon-image:not([src])
{
    list-style-image: url(data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABAAAAAQCAYAAAAf8/9hAAAA20lEQVR4Xr2RMQqEMBBFR0kjWImdlXgAL2ATr7DN7vksdps9g50XsBVyAsEyATU7ERQJkw2ysB+G/GLm5WcCvyrYTdd12tkUBG8p5Z1zPtsAtptlWaCqKrCFgzCOIxdCPNu2fdiQ8AygNE0TZFmW5HleYxIDYZcBqA1SFEWNfS/yCeu6kgClFDRNA3Ecm0owxc25A0plWUIURQdQCOFeokmhtd4Kdfg0TQ8/DIMbYDZuDxsxxownk4YWwFsIuJzgfHoB1DABoL+RBNgg7HMnUEp9u93/hL7v4e/6ABRB9HkHB7uyAAAAAElFTkSuQmCC) !important;}

.tabbrowser-tabs tab[busy] .tab-icon-image
{ 
    list-style-image:none!important;
    background: url("chrome://global/skin/icons/loading_16.png") !important; 
}