Problems with MathML rendering in Firefox 3

The issue described here is addressed by Gentoo Bug #231746.

The Problem

On my system, a Gentoo Linux, Firefox used the clubs suit symbol instead of a vertical lines in mathematical formulas.

The MathML construct

<math display="block" xmlns="http://www.w3.org/1998/Math/MathML">
 <mfenced open="{" close="}"><mrow>
  <mrow><mn>2</mn><mo>&InvisibleTimes;</mo><mi>x</mi></mrow>
  <mo>|</mo>
  <mrow><mi>x</mi><mo>&lt;</mo><mn>5</mn></mrow>
 </mrow></mfenced>
 <mo>&NotEqual;</mo>
 <mfenced open="{" close="}"><mrow>
  <mrow><msup><mi>x</mi><mn>2</mn></msup></mrow>
  <mo>|</mo>
  <mrow><mi>x</mi><mo>&lt;</mo><mn>5</mn></mrow>
 </mrow></mfenced>
</math>

is rendered by your system as

2⁢x | x<5 ≠ x2 | x<5

and by my Firefox 3 (Gentoo package www-client/mozilla-firefox-3.0-r1) as

Screenshot of incorrect MathML rendering

The Solution

In my prefs.js there was this line:

user_pref("font.mathfont-family", "CMSY10, CMEX10, Mathematica1, Mathematica2, Mathematica4, MT Extra, Standard Symbols L");

It was created in response to a recommendation from the media-fonts/mathematica-fonts ebuild. Removing that entry solved the issue for me.