Problem with Mozilla in IPv6 environment
Slow response on older Mozilla/Firefox - especially on MacOS X
Ther was some bug in the Mozilla code, that slowed down the connection handling in the Mozilla codebase - discovered in October 2004. This problem has corrected in the new version of Mozilla/Firefox - upgrade your Mozilla/Firefox.
Slow response in new Mozilla/Firefox - when there is no IPv6 connectivity
Use address selection API and prefix policy database to prefer IPv4 connection as defined in RFC 3484. More info for FreeBSD at IPv6hostsfreebsd#How_can_change_the_address_selection_policy or Window XP/2003 IPv6hostswindows#How_can_change_the_address_selection_policy. Currently no support in Linux for RFC 3484 style address selection policy.
Slow response in new Mozilla/Firefox - when there is no IPv6 connectivity - workaround
If you don't have IPv6 connectivity and you want to use it you should conplain to your internet provider. To workaround the problem you can disable DNS IPv6 lookup in the latest Mozilla/Firefox:
network.dns.disableIPv6 = true.
This apparently makes Mozilla/firefox not try IPv6 with dns lookups.