The proxy (application layer firewall, circuit level proxy, or application proxy ) is a second generation firewall
“First generation firewall” incorrect. A packet filtering firewall is a first generation firewall. “Third generation firewall” is incorrect. Stateful Firewall are considered third generation firewalls “Fourth generation firewall” is incorrect. Dynamic packet filtering firewalls are fourth generation firewalls
References: CBK, p. 464 AIO3, pp. 482 -484
Neither CBK or AIO3 use the generation terminology for firewall types but you will encounter it frequently as a practicing security professional. See http://www.cisco.com/univercd/cc/td/doc/product/iaabu/centri4/user/… for a general discussion of the different generations.