I am not sure this affects all IDN names or not, but we are running a new site at http://สภากาแฟ.net i.e. http://xn--12c4db3b2bb9h.net and all Firefox users cannot post any reply. I have isolated the problem by adding a CNAME and create new URL at http://test.serithai.net that point to the same Xenforo installation, and it works fine on my Firefox while the IDN's URL still not working. Feel free to try at the specified URLs if you need to. Edit: I set network.IDN_show_punycode to true in Firefox's about:config and it works.