alert(document.getElementById("flvplayer"));
//=> [object XPCNativeWrapper [object HTMLEmbedElement]]
alert(unsafeWindow.document.getElementById("flvplayer"));
//=> [object HTMLEmbedElement]
alert(unsafeWindow.document.getElementById("flvplayer") === document.getElementById("flvplayer"));
//=> false
alert(unsafeWindow.document.getElementById("flvplayer") === document.getElementById("flvplayer").wrappedJSObject);
//=> true

ちょっと前に GetVariable でごにょごにょしてたときハマった……(XPCNativeWrapper 経由だと呼べない)