Some users have reported that having IPv6 DNS enabled causes a slower response in domain name resolution. I would bet this will vary widely user to user depending on the capabilities of your ISP's DNS servers. However, since IPv6 isn't that useful at this point in time I have set this preference to disabled by default. Hopefully that will work out well as many people may not even be aware of this preference and that they can change it. This way they won't have to worry about it

If anyone wants to change it back just open about:config and toggle network.dns.disableIPv6 to false. I will be setting up 1.5.0.5 this way. Unless anyone knows any good reasons not to.