Clients on Orbi Wi-Fi receive an IPv6 default gateway from an Orbi unit

Disabling IPv6 on the Firewalla router does not mean that no IPv6 packets can exist on the home network (LAN).  It only means that the Firewalla does not participate using IPv6.

As StephenB​ pointed out, fe80::ca10:2fff:fe40:e8a5 is a link local address created by a device on the network that does support IPv6.  This link local address was not created by the Firewalla or by the Orbi.  Likewise, the other IPv6 addresses are coming from some device on the network.  The Orbi is not "advertising" them.