Displaying Mathml equations

Including

<script type="text/javascript" src="https://cdn.mathjax.org/mathjax/latest/MathJax.js?config=MML_HTMLorMML"></script>

and then using

    <math xmlns="http://www.w3.org/1998/Math/MathML">
        <mroot>
            <mrow>
                <mi>x</mi>
            </mrow>
            <mn>4</mn>
        </mroot>
    </math>

works in Chrome and Firefox

Update:

Since the mathjax cdn is shutting down, change

<script type="text/javascript" async
  src="https://cdn.mathjax.org/mathjax/2.7-latest/MathJax.js?...">
</script>

to

<script type="text/javascript" async
  src="https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.1/MathJax.js?...">
</script>

as others have said you need to serve as xml for FF3, if you want to use mathml-in-html5 served as text/html you need firefox 4 (or webkit nightlies)