I use both. IE for "safe" browsing (sites that Im familiar with), and Firefox for surfing (everything else). Although no browser is truly "invincible," IE seems more vulnerable to junk. Of course certain sites are written with IE in mind, and wont display correctly on firefox. Since theres no way to be 100% covered, you definitely want decent spam/virus scanners. I use multiple programs, since they all use different sets of definitions (and detect different things).