Firefox supports IPv6 by default, which may cause connection problems on certain systems. To disable IPv6 in Firefox:
1. In the Location bar, type about:config and press EnterReturn.
* The about:config “This might void your warranty!” warning page may appear. Click I’ll be careful, I promise!, to continue to the about:config page.
2. In the Filter field, type network.dns.disableIPv6
3. In the list of preferences, double-click network.dns.disableIPv6 to set its value to true.