Hi,
the reason of such behaviour in FireFox 3 is dysplay style of main table. It is "inline-block". You just need to remove it or set it to 'table'. The most simple way to do it is adding small java-script code at the end of your page with calendar control:
<
script type="text/javascript">
var c = document.getElementById('<%=CalendarCtrl.ClientID%>');
if(c!=null)
{
c.style.display = null;
}
</script>
Hope this helps.