The terminology here is as follows:
- Firewall is a layer 3 routing device which can do access control and deep inspection (amongst other things).
- IPS is a layer 2 non-routing (transparent) device which can do access control and deep inspection.
Both have access rules that allow/deny traffic based on IP or transport protocol headers. So the IPS or firewall policy element has same function for both types of devices. Big difference is that IPS access policy allows traffic by default, while FW policy denies it.
Filtering based on signature, or flow content, is what we call "deep inspection". It can be done on both IPS and firewall engines. In both, it is controlled with element "inspection policy", which is referred to on the inspection tab of both FW and IPS policies.
Difference between firewall and IPS devices is not in their ability to do inspection, but in their deployment (L3/L2) and their default action (deny/allow).