Look at the page source from within firefox and you will see that there is a
<tt> that should not be there:
<h2 class="posttitle-page"><a href="http://www.mediacentrum.nl/wordpress/gallery-2/">Photo albums</a></h2>
<div class="postcontent"><p><tt></p> <div class="wppa-nav wppa-box wppa-nav-text">
<span class="wppa-black"; >Photo albums</span> </div>
I have seen this before, strange themes... This tt tag stayes effective the whole page and overrules your Arial's that are in place.
So, find out how to remove the tt tag and it will probably be fixed.
Note that the
<p><tt></p> combination is illegal anyway, so do not expect that different browsers react the same.
By the way, did you remove class wppa-nav-text from the breadcrumb and leaving a superfluous ; there? see:
<span class="wppa-black"; >