A Firewall can’t prevent viruses attached to emails, and cannot determine the content of a fraudulent email, so you have to be careful of suspicious emails and you should have an anti-virus to make sure that received emails are not malicious before opening them. Also, firewalls can’t prevent phishing scams that are used to trick computer users to steal their personal or financial information, as phishing scams starts with an email asking for your account number, username and password.