Make sure you understand your security policy and how the firewall enforces it. Don't trust someone else to do the job for you. For one thing, your security policy will probably change over time. For another, the kind and probably the level of threat to your networks will change. You must be able to reconfigure it quickly and correctly if you're going to escape being battered by hostile forces.
Some hostiles are military professionals in the service of sworn enemies.
Some are children. They got past the best defenses we could muster. It's my own fault, I left our systems wide open. Beware of the dark side.