@namespace url(http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul);

#tabs-newbutton {
list-style-image: url(chrome://browser/skin/Toolbar-small.png) !important;
-moz-image-region: rect(0px, 160px, 16px, 144px) !important;
}

#tabs-newbutton:hover {
-moz-image-region: rect(16px, 160px, 32px, 144px) !important;
}