Maybe it is set to using ipv6 and your provider doesn't support that yet. Enter about
:config in the address bar, press Enter. In the filter box, type ipv6; only one entry (disable ipv6) should be left. If that's set to 'false', double-click it to set it to 'true'. Restart Firefox. Hope the problem goes away.
If it doesn't you might weant to revert this.
Robin