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

toolbarbutton.bookmark-item[label$="Posterous"] {
   list-style-image: url("http://posterous.com/images/favicon.png") !important;
}
toolbarbutton.bookmark-item[label$="FontFriend"] {
   list-style-image: url("http://somadesign.ca/favicon.ico") !important;
}