IPv6

Internet Protocol version 6 (IPv6) can cause connection problems on some Internet connections. Firefox uses IPv6 by default. Disable IPv6 to see if this solves your connection issues:

   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 displayed list, type network.dns.disableIPv6 in the Filter textbox.
   3. Find the entry for network.dns.disableIPv6.
   4. If the value is false, double-click on it, to change the value to true.
          * If the value is already true, IPv6 is already disabled, so it isn't causing your problem. 
   5. Close Firefox, start it again, and try to reach a web site. 

If disabling IPv6 doesn't resolve the problem, re-enable IPv6 by setting the preference back to false.


Link : http://support.mozilla.com/en-US/kb/Firefox+cannot+load+web+sites+but+other+programs+can