var favicon_container = document.createElement('div');
favicon_container.style.cssFloat = 'left';
favicon_container.style.minWidth = '16px';
favicon_container.style.minHeight = '16px';
favicon_container.style.backgroundImage =
'url("chrome://global/skin/tree/item.png")';

を、

var favicon_container = document.createElement('div');
favicon_container.setAttribute("class","favicon_container");
GM_addStyle(<><![CDATA[
.favicon_container{
float: left;
min-width: 16px;
min-height: 16px;
}
:-moz-broken:after{
content: " ";
display: inline-block;
width: 16px;
height: 16px;
background-image: url("chrome://global/skin/tree/item.png");
}
]]></>);

に変更する