Re: ARPspoof bypass firewall
You should read more about ARP poisonning. You just send out ARPs to convince victims your are the router, so I don't think there is anyway to bypass anything. If the AV checks for change in the ARP tables or someting like that, I believe there is not much you can do with arpspoof !
Re: ARPspoof bypass firewall
Some firewalls use ARP Spoof Protection as it will scan the packets for that sorta thing, for example, Comodo Firewall has the Option to protect against arp spoofing, but it's not enabled by default. So maybe the victim/mock machine hasn't enabled it? or has it enabled xD