Is there any "added value" to run a full node behind a router or firewall?
benefits for the network


Maybe, but barely.

If your node happens to connect out to a number of reachable nodes that aren't well connected by themselves, and you have a fast connection to both, it is possible that you improve relay of transactions and blocks between them. This is unlikely to be significant on itself, as reachable nodes can typically accept many connections, so there are many chances for other nodes than yours to do this as well. However, the effect of a large number of node operations choosing to do this or not may be significant.


  benefits for the individual


Absolutely, and in my opinion, this is the only reason you should (or shouldn't) run a full node.

It makes you independent.

If you run a full node, and use it to validate incoming transactions, nobody can send you an invalid transaction. There are other ways of accomplishing the same, but they all rely on trusting other parties.

And even if you don't use it to verify incoming transactions that affect you, it makes sure that miners cannot change the rules of the network without you knowing.