From MozillaZine Knowledge Base
IPv6 is a new version of the Internet Protocol that will be used in the future instead of IPv4. Not all web servers correctly support IPv6, and this can lead to connection failures and delays in loading websites. This preference controls which servers to use IPv4 on even if IPv6 is enabled.
Possible values and their effects
A comma-seperated list of domains to connect with IPv4 instead of IPv6. Default value is .doubleclick.net; in trunk builds this has been changed to an empty string.
The default setting for this preference does not mean that Mozilla will ever contact doubleclick.net (an internet ad company) if it is not requested, it just specifies which IP version to use if it is requested. The .doubleclick.net address was originally added because their DNS server was broken and caused delays in page loads; this has since been fixed. If you wish, you may delete this preference with no ill effects.
First checked in
Has an effect in