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

*[item-command="auth"] {
    list-style-image: url("img/twitter-locked.png") !important;
}
*[item-command="slice"] {
    list-style-image: url("img/twitter.png") !important;
}
*[item-command="slice"][twitter-fresh="true"] {
    list-style-image: url("img/twitter-fresh.png") !important;
}