I am guessing that IPv6 slows down web connections because most web servers aren’t using it yet.
http://www.tweakxp.com/article37127.aspx
i think your router has a problem with ipv6 support (i have the same problem with my zyxel router by the way).
you can turn off ipv6 support in mozilla firefox by typing about:config in the firefox address bar. this should open the options page for firefox. use ipv6 as filter or scroll down to the network.dns.disableIPv6 option, doubleclick on it so it’s set to true
if this fixes the slow response time in firefox, then ipv6 support is indeed the problem. there is a way to completely disable ipv6 (not only in firefox), but i don’t remember how right now.
To disable IPv6 you add the following line to the bottom of /etc/modprobe.d/blacklist
blacklist ipv6
you will need to be root to do this.
http://ubuntuforums.org/showthread.php?t=93164