Prior to 0.8, Firefox did not support IPv6. Since then, IPv6 has been supported by default and is preferred over IPv4. Firefox also requests AAAA DNS records even when there is no IPv6 connectivity.
To be certain that the IPv6 protocol is being used to access a website, substitute an IPv6 literal address surrounded by square brackets in place of a domain name. For example.
For additional information, refer to Request For Comments (RFC) 3986 “Uniform Resource Identifier (URI): Generic Syntax".
(Caution: This substitution can sometimes fail. Explanation of possible reasons for this are available. If you encounter problems, review the Broken User FAQ article found on that website for several possible explanations. For even more possible explanations, review this article on the ARIN IPv6 wiki.)
To disable IPv6 support in Firefox:
1. In the Location bar, enter about:config. Then press the Enter or Return key
2. If the "This might void your warranty!" warning page appears, click I'll be careful, I promise!
3. In the Search preference name field, type network.dns.disableIPv6
4. network.dns.disableIPv6 should now appear in the list of preferences. Click on the Toggle button to set its value to true
5. Close the preferences
To re-enable IPv6 support in Firefox after it has been disabled, repeat steps 1 through 3 above. Then:
4. Set the value of network.dns.disableIPv6 to false
5. Check the value of network.dns.ipv4OnlyDomains. If it contains any domain names, delete them.
6. Close the preferences
Extensions: IPvFoo is one of several extensions that are now available for FireFox. After you install iPvFoo, some combination of 4, 6 and/or "?" will appear at the top of the screen near the menu button (). Clicking on it will cause the IP address(es) of the web page you are viewing to appear. To install iPvFoo, open the Extensions manager, enter "IPvFoo" (without the quotes) in the Find more add-ons box, and click on IPvFoo. FireFox has many other extensions that are specific to IPv6.