Firefox 3 by default resolves IPv6 address first and if that fails then reverts to IPv4. This can sometimes slow down your browser’s performance.
Here are the steps to turn off IPv6 support in Firefox 3 and above.
1. Key in “about:config” into address bar, this will take you to the config page.
2. Key in “network.dns.disableIPv6″ into the filter bar and hit enter.
3. Right click on the line that read “network.dns.disableIPv6″ and select the “Toggle” option. This will switch the value from “false” to “true”
4. Restart the browser and enjoy the improvement.