The latest firmware is useful from a security and functionality point of view but there's no need to use BT specific firmware for FTTP connections.
I don't think I've even tried the Fritz!Box router that came with the Zen connection for FTTP. I'm using a Watchguard firewall with the external interface set to authenticate with PPPoE. It works with DHCP or with static IP address explicitly set on the firewall. I've got a range of static IP addresses and can happily route to them as expected.
I'll be playing with IPv6 next week and will let you know how I get on.