Smallwall (WAN-interface) seems not to announce its routing information to the FRITZ!Box via router advertisement. (wireshark sees only a neighbor sollicitation) The (random?) subnet gets on smallwall.
Smallwall blocks all ipv6 traffic (logs) although a pass rule exsists
I've used IPV6 for years on Monowall. It appears to be identical in SmallWall. I haven't implemented as of yet since my monowall box is off-site at the moment (recently moved and haven't moved data centers yet).
Do you have native V6 or are you doing some sort of tunneling?