Doubt on showing MathML in Firefox

I am trying to open the following local file in Firefox and it doesn't show
the formula correctly. My web connection is OK. When I try to open web pages
that use mathml they show the formulas correctly. Do you have any idea of
why the problem appears just with local files?

Regards,

Romeu

HTML file ("<" and ">" replaced by "[" and "]"):

[?xml version="1.0"?]
[?xml-stylesheet type="text/xsl" href="http://www.w3.org/Math/XSL/mathml.xsl
"?]

[html xmlns="http://www.w3.org/1999/xhtml"]

[head][/head]

[body]

[math display="block" xmlns="http://www.w3.org/1998/Math/MathML"]
[msup]
[mfenced]
[mi]a[/mi]
[mo]+[/mo]
[mi]b[/mi]
[/mfenced]
[mn]2[/mn]
[/msup]
[/math]

[/body]
[/html]