From MozillaZine Knowledge Base
IPv6 was designed in part to solve the problem IPv4 will soon be facing: the exhaustion of all possible IP addresses. Mozilla implemented IPv6 support in early 2000, but that support did not receive widespread testing until recently as IPv6-capable OSs and network software/equipment became more common.
One particular bug that has appeared exists not in Mozilla, but in IPv6-capable DNS servers: an IPv4 address may be returned when an IPv6 address is requested. It is possible for Mozilla to recover from this misinformation, but a significant delay is introduced.
Under certain versions of Mac OS X (those prior to 10.4), this bug is compounded by another bug wherein the OS still makes IPv6 DNS requests even if IPv6 support is disabled. A significant delay is introduced in all connections requiring DNS lookups while the OS and the DNS server exchange unnecessary (or redundant) queries and responses to resolve the address.
Two approaches were taken to work around these bugs: first, a preference to globally disable IPv6-based DNS lookups browser-wide (this preference), and a blacklist of domains which should never have IPv6-based DNS lookups performed (network.dns.ipv4OnlyDomains).
Possible values and their effects
Disable IPv6 DNS lookups. (Default in Mac OS X prior to 2008-01-05, OS/2)
Perform IPv6 DNS lookups on IPv6-capable OSs. (Default everywhere else)
If your OS or ISP does not support IPv6, there is no reason to have this preference set to false. Some suggest that setting this preference to true on any OS will speed up connections, though this is probably a placebo effect.
First checked in
Has an effect in