Problema com a grid no firefox

Olá pessoal, estou com um problema num codigo javascript, que gera uma grid, fucniona normalmente no internet explorer, mas no firefox e chrome da erro is not defined:

Usando o firebug do firefox, ele da seguinte mensagem de erro:

var cs=ComputedCSSStyleDeclaration.prototype grid.js (linha 93 )

ReferenceError: ComputedCSSStyleDeclaration is not defined

A função de onde ele indica o erro:

function(){
if(!window.HTMLElement){

return}

var element=HTMLElement.prototype;
element.__proto__={__proto__:element.__proto__};
element=element.__proto__;
var capture=["click","mousedown","mouseup","mousemove","mouseover","mouseout" ];
element.setCapture=function(){
var self=this;
var flag=false;
this._capture=function(event){
if(flag){
return}flag=true;
self.dispatchEvent(event);
flag=false};
for(var i=0;i<capture.length;i++){
window.addEventListener(capture[i],this._capture,true);
window.captureEvents(Event[capture[i]])}};
element.releaseCapture=function(){
for(var i=0;i<capture.length;i++){
window.releaseEvents(Event[capture[i]]);

window.removeEventListener(capture[i],this._capture,true)}

this._capture=null};
element.attachEvent=function(name,handler){
if(typeof handler !="function"){

return}

var nsName=name.replace(/^on/,"");
var nsHandler=function(event){
window.event=event;
handler();
window.event=null};
handler[name]=nsHandler;
this.addEventListener(nsName,nsHandler,false)};
element.detachEvent=function(name,handler){
if(typeof handler !="function"){

return}

var nsName=name.replace(/^on/,"");
this.removeEventListener(nsName,handler[name],false);
handler[name]=null};
var getClientWidth=function(){
return this.offsetWidth - 20};
var getClientHeight=function(){
return this.offsetHeight - 20};
element.__defineGetter__("clientWidth",getClientWidth);
element.__defineGetter__("clientHeight",getClientHeight);
var getRuntimeStyle=function(){
return this.style};
element.__defineGetter__("runtimeStyle",getRuntimeStyle);

// Erro está aqui

var cs=ComputedCSSStyleDeclaration.prototype;
cs.__proto__={__proto__:cs.__proto__};
cs=cs.__proto__;
cs.__defineGetter__("paddingTop",function(){
return this.getPropertyValue("padding-top")});
var getCurrentStyle=function(){
return document.defaultView.getComputedStyle(this,"")};
element.__defineGetter__("currentStyle",getCurrentStyle);
var setOuterHtml=function(s){
var range=this.ownerDocument.createRange();
range.setStartBefore(this);
var fragment=range.createContextualFragment(s);
this.parentNode.replaceChild(fragment,this)};
element.__defineSetter__("outerHTML",setOuterHtml)})();

Favor quem puder me ajudar, desde ja agradeço !!