Threatfire is a behaviour blocker, and so will warn of potential malicious behaviour (hopefully) that isn't covered by a signature based application. It also has a virus database to refer to for known malicious actions.
Previous versions had some compatibility issues, the current one seems to work fine. (Quite happy with it.)
Are you using MBAM and SAS in realtime? Might get some issues, there.
If you have D+ active in Comodo, I don't think there is much need for Threatfire.
For a non-HIPS firewall, I think it is a useful addition.
Be aware that it can be registered in the security centre as an AV, so unless it is set not to (easy), the security centre will report more than one AV active.