The menclose tag of MathML is not working in Firefox (I am using version 3.0). For example, the examples 2 and 3 do not work in the following sample [copy and paste the following to a notepad and save as test.xhtml file and then open in Firefox]. These examples 2 and 3 are taken from section 3.3.9 of http://www.w3.org/TR/2003/REC-MathML2-20031021/chapter3.html
<?xml version="1.0"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1 plus MathML 2.0//EN"
               "http://www.w3.org/TR/MathML2/dtd/xhtml-math11-f.dtd" [
  <!ENTITY m "http://www.w3.org/1998/Math/MathML">
]>
<html xmlns="http://www.w3.org/1999/xhtml">
  <head>
    <title>Minimum Document with MathML</title>
  </head>
  <body>
<h3>Example 1</h3>
<p><math xmlns="&m;"><msup><mi>x</mi><mn>2</mn></msup></math></p>
<p>
<h3>Example 2</h3>
<math xmlns="&m;">
<mtable columnspacing='0pt' rowspacing='0pt'>
<mtr>
  <mtd></mtd>
  <mtd columnalign='right'><mn>10</mn></mtd>
</mtr>
<mtr>
  <mtd columnalign='right'><mn>131</mn></mtd>
  <mtd columnalign='right'>
    <menclose notation='longdiv'><mn>1413</mn></menclose>
  </mtd>
</mtr>
<mtr>
  <mtd></mtd>
  <mtd columnalign='right'>
    <mrow>
    <munder>
      <mn>131</mn>
      <mo> &UnderBar; </mo>
    </munder>
    <mphantom><mn>3</mn></mphantom>
    </mrow>
  </mtd>
</mtr>
<mtr>
  <mtd></mtd>
  <mtd columnalign='right'><mn>103</mn></mtd>
</mtr>
</mtable>
</math>
</p>
<p>
<h3>Example 3</h3>
<math xmlns="&m;">
<msub>
  <mi>a</mi>
  <mrow>
    <menclose notation='actuarial'>
      <mi>n</mi>
    </menclose>
    <mo>&it;</mo>
    <mi>i</mi>
  </mrow>
</msub>
</math>
</p>
  </body>
</html>