Edite o arquivo userChrome.css para recuperar o antigo ícone "nova guia":
/*
* Do not remove the @namespace line -- it's required for correct functioning
*/
@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* set default namespace to XUL */
#page-proxy-favicon:not(src), .tab-icon-image:not(src),
#personal-bookmarks .bookmark-item .toolbarbutton-icon:not(src) {
list-style-image: url("chrome://global/skin/icons/folder-item.png")!important;
-moz-image-region: rect(0px, 16px, 16px, 0px)!important;
}
}