A firewall can be run "in-line" and not have IP addresses on the interfaces. On a Palo Alto firewall this would be a "virtual wire", and "transparent firewall" or "bridging firewall" would be other common terms.
Examples:
https://docs.opnsense.org/manual/how-tos/transparent_bridge....
https://docs.netgate.com/pfsense/en/latest/bridges/index.htm...
https://www.fortinet.com/resources/cyberglossary/transparent...
