Mac OS X Firefox 3.5.3 and MathML arrows

Hi,
I have a strange issue with the rendering of a few MathML symbols in
the Mac version of Firefox 3.5.3 (I have the latest STIX fonts
installed). The bug can be seen at this page:

http://www1.chapman.edu/~jipsen/mathml/asciimathsyntax.html

All the symbols are rendered correctly, but for rarr, larr and harr
(nothing is displayed). I've started a thread on moodle.org about
that, but this seems to be specific of the Mac version of Firefox
(Windows Firefox displays those symbols correctly).

Can anyone confirm this is a Firefox issue?