Why is getElementsByTagName returning undefined?

I'm trying to call document.getElementsByTagName, and I'm getting back undefined as a result, no matter what parameter I pass. (Even if I pass "*".)

I tried Googling for it, but all the search results were about elements of the getElementsByTagName result array being undefined. What I'm getting is undefined as the result itself, and it's driving me up the wall.

Does anyone know what can cause this? (Using Firefox 12.0. In Chrome I get the expected results.)

EDIT: OK, here's sample code:

function buttonClick(){
   var xhr = new XMLHttpRequest();
   var msg = document.getElementById('message');
   var buttons = document.getElementsByTagName("button");
   var button, i;
   for (i = 0; i < buttons.length; ++i){
      button = buttons[i];
      msg.removeChild(button);
   }

   xhr.onreadystatechange = function() {
        if(xhr.readyState == 4){
            handleResult(xhr.responseText, msg);
        }
   };
   xhr.open("POST", location.href, true);
   xhr.setRequestHeader("Content-type","application/x-www-form-urlencoded");
   xhr.send("cmd=MyCommand");
}

And the getElementsByTagName always returns undefined, whether I trace it in Firebug's Script tab or call it from the Console tab. (Also in Firebug, since this seems to be confusing people. Apparently there are way too many consoles floating around.).

As proof, here's what I've been getting when I tried to use the Firebug console:

>>> document.getElementsByTagName("button");
undefined
>>> msg.getElementsByTagName("button");
undefined
>>> msg.getElementsByTagName
getElementsByTagName()
>>> msg.getElementsByTagName("BUTTON");
undefined
>>> msg.getElementsByTagName("*");
undefined
>>> document.getElementsByTagName("*");
undefined
>>> document.getElementsByTagName("body");
undefined

The markup is (or ought to be) irrelevant. It's a valid, well-formed HTML page with some buttons and other elements on it. This JS function is attached to the onclick of one of the buttons. But it looks something like this:

<html xmlns="http://www.w3.org/1999/xhtml"><head>
blah
</head>
<body>
<script type="text/javascript" src="/myJS.js"></script>
<div id="page-container">
   <div id="message"><button onclick="buttonClick();">Button 1</button><button onclick="ButtonClick2()">Button 2</button></div>

</div>

</body></html>

Solution

edit:

This is a bug in firebug and is fixed by upgrading to 1.10.0a7

Because it is impossible for this method to return undefined, there are 2 possibilities:

Your debugging tools are lying to you
document.getElementsByTagName is not referencing the original host object. It should print function getElementsByTagName() {[native code]} when referenced in console.

You should be able to reliably to see if it's in fact undefined (in firefox) with this:

delete window.alert;
window.alert(buttons);

The delete is a NOOP if window.alert is already referencing the original host object, otherwise it will restore it.

If it alerts undefined, you should be able to do

delete document.getElementsByTagName

to restore the host object reference.

All console references here refer to the built in Web Console that comes with firefox by default.