You don't "need" to install Firefox, but you can. It is far less safe as it comes from Mozilla than Safari in recent tests, but can be improved a lot by installing NoScript and by turning off Java (not the same as JavaScript). Safari comes with your Mac as the default browser. The only way to use Internet Explorer is to install Windows. Apple doesn't pay Microsoft for a licensing fee to include Windows with new Macs. You have to go get the Windows install DVD yourself. An application called "Mail" for sending email comes with your Mac. Look at the bottom of the screen. See that row of application icons?
These PC geeks are mistaken. There are not a handful of Macs. There are over 75 million in use. If someone makes a virus for Windows, it is small news. If someone makes a virus for Mac, it is big news. Those kids who make viruses to get headlines would love to attack Macs and make really big headlines, but they can't, because Mac OS has better protection than Windows. As far as "a hundred times as many", the ratio of Windows to OS X use is 7.274 to one.
No antivirus application comes bundled with or has ever come bundled with any Apple computer from the Apple factories.
Free: ClamXAV or iAntiVirus (latter requires OS 10.5, Intel).
Pay: Virex, Norton, Sophos, Intego, MacScan, Avast. Not one Mac owner in a million has ever experienced a virus. It is not merely that their are fewer Mac computers that accounts for fewer Mac viruses. The Mac OS does not permit any file added to the base system directories until you type your admin password. That doesn't account for any open-door policies, such as your allowing third-party cookies in Safari or allowing Java and cross-scripting. You can close all those doors and still surf the Web, thus preventing malware from settling in on your Mac.
The best for getting rid of tracking cookies is MacScan. The new version is US$29.95; if you can find version 2.6 (no longer on their site), it is free and malware profiles updates are free -- skip the MacScan engine update; that cancels your free use.
After you do all the scanning routines to get rid of tracking cookies and rare but possible other bad stuff, start using Firefox with NoScript and NoRedirect installed, uncheck the option to allow third-party cookies, uncheck the option to allow Java (different from Javascript which you likely wouldn't want to disallow as almost all sites rely on it), be sure the built-in OS X firewall is on, set the advanced options of the firewall to use stealth mode and Block UDP Traffic, use a hardware router even if you use only one computer on your network, don't keep Personal File Sharing / Windows Sharing / Personal Web Sharing / Remote Login checked when you don't absolutely need to use them, don't use Wi-Fi during any secure data exchange such as online banking or CC purchase, when you do use Wi-Fi, set a WPA password and choose to make the network hidden, never open any email from someone you don't recognize, and never open attachments unless you were expecting it (skip all the jokes and chain letters).
Some secondary means of security are available at securemac.com