getting started with MathML; clueless and having problems.

I am trying to create a .htm file with imbedded MathML and render it
with FireFox 3.0.5.  I checked that this browser was MathML enabled by
running the sample files suggested in http://www.w3.org/Math/XSL/ and
they rendered properly.

I downloaded the requested files mathml.xsl, pmathml.xsl, ctop.xsl and
pmathmlcss.xsl and copied the source of the file http://www.w3.org/Math/XSL/pmathml2.xml
onto my local machine.  I modified the stylesheet declaration in the
source to be

<?xml-stylesheet type="text/xsl" href="mathml.xsl"?>

as suggested by the  http://www.w3.org/Math/XSL/ page.  I also
installed the STIX fonts on my machine (Windows XP Professional, 2002,
Service Pack 2).

When I invoke the modified source file, the equations do not render
properly.  Specifically the simple example with brackets and
superscripts renders as a+b260+a+bi.

I have almost no information to begin diagnosing the problem, so I
appeal to wiser minds to give me a gentle pointer to where to start.

Thanks.

  Jack Gjovaag