That's completely the wrong approach..
If your hosts aren't secure enough to be on the public internet, they shouldn't be on an internal network either. Many attacks come from the inside, and if you have a large number of insecure hosts hidden behind a border firewall then all it takes is one tiny hole and everything can come crashing down, as has happened many times in the past.
A firewall is not the ultimate answer, and nor should it be your only line of defense. If hosts are correctly configured, then a firewall won't actually improve security as the only services exposed on the host will be ones you intended to run and thus explicitly allowed through the firewall.