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

menuitem[disabled="true"],
menuitem[disabled="true"] + menuseparator
 { display: none; }

/*
to hide disabled menuitems on context menu only, use:

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

#contentAreaContextMenu menuitem[disabled="true"],
#contentAreaContextMenu menuitem[disabled="true"] + menuseparator
 { display: none; }
*/