JQuery :: Effect Does Not Run In Firefox

I'm doing, like usual, an amazing job of outstanding webdesign, suddenly I told my self. Let's try this piece of art in Firefox doesn't work! (In safari and chrome it's ok. I don't have it, but I bet that in IE doesn't work too!) I've made some tabs that are addressed to show some content when clicked; here is the script I've made:

$(document).ready(function () {
$('#zona1 .sez.active').fadeIn(0);
$('#schede li span.centro').click(function(){
var sezione = $(this).attr('id')
if (this.id != 'active') {
$('#schede li span.active').removeClass('active').parent().removeClass('active');
$(this).addClass('active');
$(this).siblings().addClass('active');
$(this).parent().addClass('active');
$('#zona1 .sez.active').fadeOut(400, entrata).removeClass('active');
function entrata (){
$(sezione).fadeIn(300).addClass('active')
};
};
});
});

If you want to have a look to the code and the page just click:
800.omnigrafica.it/storia.html