Why is this method "undefined" in Firefox once the page has loaded?
<pre><code>"use strict";
Math.isNumber = function(number) {
    return (number &gt;= 0 || number &lt; 0);
};
</code></pre>

<p>I have placed the above in a clean HTML file within <code>&lt;script&gt;</code> tags within the <code>&lt;head&gt;</code>. Before the page has fully loaded, I can use <code>alert(Math.isNumber("blah"))</code> and I get a value. But once the page has loaded, typing <code>Math.isNumber</code> into Firefox's web console yields <code>undefined</code>. In Chrome, it returns the function I set above. Why?</p>
