It's technically possible for nftables and iptables to coexist, but this might be a headache to manage.

See here for more info: https://unix.stackexchange.com/questions/596493/can-nftables-and-iptables-ip6tables-rules-be-applied-at-the-same-time-if-so-wh

Firezone creates a separate nftables table (firezone) and confines any firewall changes to that table. So at worst, Firezone won't mangle your current rules.

I'd recommend using iptables-nft which will provide an iptables-compatible frontend to the kernel's netfilter system. Apply your rules using that, and you'll have much better luck getting Firezone to play nicely.

Keep in mind Firezone adds default routes and enables IP packet forwarding as well which may affect any existing rules you have.

Closing as outdated