Let's say I have an HTML5 document with some MathML in it. I set the font of the body of the page but it doesn't change the font used for MathML characters:
<title>Example of MathML embedded in an HTML5 file</title>
<meta charset="utf-8" />
<body style="font-family: courier;">
You should probably read this document on the Mozilla Developer Network. I'm guessing the following paragraph is applicable in your case:
Mathematical formulas make use of various symbols represented by specific Unicode characters. Mozilla can display any of these symbols provided suitable Unicode fonts are installed. Furthermore, in accordance with the W3C CSS2 recommendation on fonts, authors can specify an ordered list of particular fonts which they prefer (using the font-family property of CSS), with the assurance that Mozilla's font engine will hunt for alternate fonts whenever their specified fonts are not found on a particular user's system.
One of the Mozilla
MathML developers seems to blog quite regularly. There are two recent posts which will be of interest to you: