Here is a MathML sample code I am using to test MathML rendering.
Demo URL: http://jsfiddle.net/3ak4P/
<!DOCTYPE html>
<html lang="en">
<head>
<title>MathML demo</title>
<style type="text/css">
math {
display: block;
font-size: 16px;
}
</style>
</head>
<body>
<math>
<mrow>
<munder>
<mo>∑</mo>
<mrow>
<mi>p</mi>
<mtext> prime</mtext>
</mrow>
</munder>
<mi>f</mi>
<mo stretchy="false">(</mo>
<mi>p</mi>
<mo stretchy="false">)</mo>
<mo>=</mo>
<msub>
<mo stretchy="false">∫</mo>
<mrow>
<mi>t</mi>
<mo>></mo>
<mn>1</mn>
</mrow>
</msub>
<mi>f</mi>
<mo stretchy="false">(</mo>
<mi>t</mi>
<mo stretchy="false">)</mo>
<mo> </mo>
<mo mathvariant="italic">d</mo>
<mi>π</mi>
<mo stretchy="false">(</mo>
<mi>t</mi>
<mo stretchy="false">)</mo>
</mrow>
</math>
</body>
</html>
Output with Firefox 8 on Windows XP:

Output with Firefox 8 on Debian GNU/Linux:

Now, considering that one can’t insist the users of a website to install new fonts, etc. what are the possible ways to ensure that Windows users also have a good experience browsing math formulas written with MathML?
Here’s an answer from the main guy working on Firefox MathML now (Frédéric Wang), though I doubt it will make you happy in the short-term:
Users need to install STIX or Asana fonts to get a decent MathML rendering. Maybe one can use downloadable fonts to force these fonts to be used on a given Web page, but I never tried.
Two bugs that could be interesting: