I must strongly disagree with Avant and Slimbrowser.
Here is the logic behind that statement: Both of those are "skins" that sit on top of Internet Explorer and change the look/feel and add additional functionality, but the underlying browser is still Internet Explorer... with all of the security holes of Internet Explorer. That is also why those two "browsers" display things just like IE does. Because they ARE IE.
There are no less than 7 security holes in IE right now that <font color="red"> there is no current fix for. </font> Some of those vulnerabilities have been known for over 8 months and MS still has not issued a patch. Most spyware and otherwise "bad" software requires user interaction, or tricking the user to get onto a PC. But a fair amount of those use those vulnerabilities and have the ability to load software just from visiting a site or receiving an email with a hyperlinked image or javascript to the "bad" site.
People depend too much on antivirus and spyware blockers to protect them and to be conscientious about security they should be looked at not as the defense, but as the LAST defense. If your antivirus or spyware software catches something, then something ELSE went wrong, because it made it all the way on to your PC. Close the hole that let it get there, or train the user that clicked on the scam link. With the currently unpatched holes, using email software other than Outlook or Outlook Express, and using IE only to get software updates from Windows Update, unfortunately is the only way to truly fix that problem until all holes are fixed.
Don't get me wrong. ANY and ALL software has security problems. The big difference with Firefox, is that if a security problem is found. It is usually fixed in the nightly update if you run the development branch and a fix for security problems is usually no more than 24 hours away.