Toolbar icons not showing properly in firefox

Hello,

First of all, my congratulations for this excellent product!!
I'm testing FCKEditor and I'm having some troubles in order to make it work properly under firefox. While in IE all is working fine, when I change to Firefox I see toolbar icons with a not-found image icon above the proper image. You can find a test example in this URL:

http://www.poveravoce.net/cefoac_new/in ... menu_id=45

Many thanks!