A firewall can’t really protect against threats within your network. Can not prevent individual users with modems from dialing in or out of the network, bypassing the firewall altogether, employee misconduct or carelessness can not be controlled by firewalls.
A firewall cannot protect against hacking from within. A firewall is just another layer of security for things like:
Viruses that spread via email, such as Trojan horses, which disguise themselves as useful or benign software and trick you into opening or downloading Trojan horses. Similar to the virus threat, it is easy to trick a user into downloading a program or opening an email attachment that may allow malicious code into the local system.
Spyware, as they can enter your computer in numerous ways such as when you open web pages (cookies / ActiveX) or when you install a program that comes with adware / spyware, so having your ports closed doesn’t matter.
Malware is a combination of the words malicious and software and includes viruses, trojans, worms, spyware/adware, phishing, and pharming. The most common way of getting malware is by opening email attachments and clicking on the URLs that are present in E-Mail. Viruses, Trojans, and worms can do a lot of bad things to your computer or business.
Social engineering, this term has come into vogue in recent years to describe the methods hackers use to obtain information from “friends” users, physical outrages such as cutting a circuit, shortage of other power sources or failures etc wireless network without warranty. The firewall does not prevent access to an otherwise unsecured wireless network.
However, the firewall helps to protect the computers on your network, so that if an intruder did gain access to the network, he or she could not access your personal computer.