"attr(id) special check failed" jQuery
<p>I receive this message from the console:</p>

<blockquote>
  <p>attr(id) special check failed Insted of accessing the property via<br>
  attr(id), use undefined instead.</p>
</blockquote>

<p>My code is really simple:</p>

<pre><code>$(document).ready(function(){
    var sezione = "";
    $('.menu_bar').click(function() {
        sezione = $(this).attr('id');
    });
});
</code></pre>

<p>What could be the problem?</p>

<p>EDIT</p>

<p>This is the html in my page:</p>

<pre><code>&lt;html xmlns="http://www.w3.org/1999/xhtml"&gt;
&lt;head&gt;
&lt;meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /&gt;
&lt;link rel="stylesheet" type="text/css" href="/css/glamStyle2.css" /&gt;
&lt;script src="http://www.modernizr.com/downloads/modernizr-latest.js"&gt;&lt;/script&gt;
&lt;script src="http://code.jquery.com/jquery-latest.js"&gt;&lt;/script&gt;
&lt;script src="http://ajax.googleapis.com/ajax/libs/jqueryui/1.8/jquery-ui.min.js"&gt;&lt;/script&gt;
&lt;script language="javascript" type="text/javascript" src="/js/jquery.easing.1.3.js"&gt;&lt;/script&gt;
&lt;!--[if IE]&gt;&lt;script src="excanvas.js"&gt;&lt;/script&gt;&lt;![endif]--&gt;
&lt;script src="/js/jquery.jsPlumb-1.3.1-all-min.js"&gt;&lt;/script&gt;
&lt;script language="javascript" type="text/javascript" src="/js/floating2.js"&gt;&lt;/script&gt;
&lt;title&gt;Glam Group srl&lt;/title&gt;
&lt;/head&gt;

&lt;body&gt;
  &lt;div id="container"&gt;
    &lt;div id="top_bar" class="menu_bar"&gt;&lt;/div&gt;
    &lt;div id="left_bar" class="menu_bar"&gt;&lt;/div&gt;
    &lt;div id="right_bar" class="menu_bar"&gt;&lt;/div&gt;
    &lt;div id="bottom_bar" class="menu_bar"&gt;&lt;/div&gt;
  &lt;/div&gt;
&lt;/body&gt;
&lt;/html&gt;
</code></pre>

<p>I still don't understand the problem...</p>

<p>EDIT 2</p>

<p>This happen only within the firebug console in Firefox. I don't have this problem with the Chrome console.</p>
