I am dismayed that you think you can't get viruses and malware using Firefox. Don't get me wrong, Firefox is probably the better browser, but you are setting him up with a false sense of security.
The majority of malware is specifically targeted at IE though this may change as market penetration by other browsers increases.
That Firefox is a vastly safer browser is beyond question;also where vulnerabilities are found and exploited FF is very quick to offer a solution.It has been said that FF on its own is about as safe as IE with an average antivirus program. Nothing is safe in this world and FF can suffer attack.However there are other ways computers can accept malware such as installing programs from dodgy sites , P2P use and by opening e mail attachments.
There's a very useful website called Annoyances.com which deals with computer problems.It's a forum which has contributions from many experts.It regards IE as a malware magnet.