For a firewall device It is quite common to stress test the throughput capacity and simultaneous connections (and in some scenarios even the maximum size of the NAT translation table).
Then there is the amount of new connections per unit of time the firewall device can establish (rate of new connections).
This parameter is of huge significance in a firewall and in my experience, not many people talks about it and as a consequence, not many people do test it.
In fact, a couple of times I have seen firewalls fell on their knees under a DDoS that is well under the maximum advertised throughput of the firewall but which has a very high connection rate (last I remember above ~100Kconns/sec).
When this happens the explaining is not always easy to do:
"What do you mean with a 100Mbps DDoS our firewall fell???, isn't it supposed to handle Zillions of Gbps of throughput??? I want to talk with the sales engineer right know dammit!"