javascript cancelBubble does't work on firefox
<p>the blow code work fine on IE,but does't work on firefox,why? It's some problem on my code? How to fix it? thanks!</p>

<pre><code>&lt;html&gt;
&lt;head&gt;
&lt;style&gt;
body{font-family:Arial;font-size:12px;font-weight:normal;line-height:28px;}
.product_tips{
width:500px;
background:#f0f0f0;
border:1px solid #ccc;
padding:8px;
margin-top:3px;
}
span{cursor:pointer;}
&lt;/style&gt;
&lt;script type="text/javascript"&gt;
    function get(id){ 
        return document.getElementById(id);
    }
    function showTip(e){
        if(get("product_tips").style.display == "none"){
        get("product_tips").style.display = "block";
    } else{
        get("product_tips").style.display = "none";
    }
    stopBubble(e)
}
function stopBubble(e) {
    if (e){
     e.stopPropagation();
     }
    else{
     window.event.cancelBubble = true;
    }
}
document.onclick = function(){
        get("product_tips").style.display = "none";
    }
&lt;/script&gt;
&lt;/head&gt;
&lt;body&gt;
&lt;div class="relative_"&gt;
&lt;label&gt;&lt;input type="text" name="#" value="" id="product_name" maxlength="6" /&gt;&lt;/label&gt;&amp;nbsp;&amp;nbsp;&lt;span onclick="showTip();"&gt;help ?&lt;/span&gt;
                &lt;div id="product_tips" class="product_tips" style="display:none;" onclick="stopBubble();"&gt;
                    &lt;div class="product_inbox"&gt;
                        &lt;p&gt;content content content content content content content content content content content content content content content content content content content content &lt;/p&gt;
                    &lt;/div&gt;
                &lt;/div&gt;
                &lt;/div&gt;
&lt;/body&gt;
&lt;html&gt;
</code></pre>

<p>the demo here : <a href="http://jsbin.com/ivosa3" rel="nofollow">http://jsbin.com/ivosa3</a></p>
