PostArticles.aspx Bug In Firefox

If you try to click the logout button in Firefox a javascript error is thrown which prevents the javascript postback call from completing.

This only happens on the PostArticle.aspx page.

The error is: 
Error: document.all is undefined
Source File: http://www.dotnetfunda.com/post/?ReturnUrl=/post/postarticle.aspx
Line: 207

The code it has a problem with is:

    function ValidateCheckBox(source, args)
    {
        if(document.all["ctl00_PlaceHolderContent_chkAgreed"].checked == false)
            {
            alert("You must agree (check the checkbox) to proceed.");
            args.IsValid = false;
        }
    }

Which looks like a custom validator but I don't know which checkbox it is trying to validate.
Document.all is an IE only dom technique.

http://www.google.co.uk/search?q=document.all+firefox