For Eg.
<?xml version="1.0"?>
<?xml-stylesheet type="text/xsl" href="mathml.xsl"?>
<html xmlns="http://www.w3.org/1999/xhtml">
<head>...</head>
<body>
<h1>Example</h1>
....
<math xmlns="http://www.w3.org/1998/Math/MathML">
<mi>x</mi><mo>+</mo><mn>3</mn>
</math>
</body>
</html>
If the above file is named as mathml.xml, then firefox displays the mathml correctly, but not when its named as mathml.html.