strange
DNS is working fine, FireFox should not have a problem with it

anyway try disabling IPv6 from FireFox and than try browsing

1. Type about:config in the Firefox address bar.
2. Type network.dns.disableIPv6 in the Filter box.
3. Double-click the network.dns.disableIPv6 entry to change it from false to true.

and you can also give some other browser like Opera Opera Web Browser | Faster & safer | Download the new Internet browsers free a shot