Not exactly open firmware, but something like ONVIF on an isolated network is pretty close. An ONVIF camera should talk to basically any NVR (network video recorder), there are several open-source ones available.

There are tons of ONVIF-compliant and cheap PoE cameras, and with this setup you really don't care about the security of the manufacturer firmware since you aren't exposing them to the network publicly. However, you do need to be extra diligent when configuring your VLAN/network segmentation.