How to bypass/hack a Firewall

As we know that Firewall is a basic component for securing a network.The basic purpose of a firewall is to isolate one network from another. Firewalls are becoming available as appliances, meaning they are installed in to the network between two networks.The Hackers often bypass Antiviruses and Anti-spywares by some methods Like Crypting,Hexing, File-pumping etc,but it becomes very difficult for