Probably yes, but it depends on what exactly you mean by "behind". To a lesser degree, it also depends on what management version you are running and what firewall version you are running.
For most useful definitions of "networks behind a firewall", the routing table is the way to go. The GAiA API can dump that for you for non-VSX firewalls. No management involvement needed, but the GAiA API is relatively new. While I use VSX a lot, I have not yet tried using the GAiA API together with VSX, so I don't personally know whether they can work together.
For some definitions of "networks behind a firewall", you will need the antispoofing topology, which you can get via the management API. A few simple API commands ('show gateways-and-servers' to find the firewall you want, then 'show object uuid ____' to get the contents of the object) should give you all the raw data you need, though some quick jq would pare it down to only the topology. These API commands are available from R80 up, but I don't know if they return enough information about the firewall's interfaces to find the antispoofing topology in earlier versions. I know in R80.20 and up, they do.