<i>but in the worst case IE is probably tied with Firefox as the most secure browser.</i><p>The difference being that<p><i>Once the malware is downloaded and installed, it opens a back door that allows the attacker to perform reconnaissance and gain complete control over the compromised system.</i><p>is less feasible in Firefox than it is in IE. Firefox forces you to download an executable and then manually run it, so a user would be conscious that he or she is installing a program. It is still possible (last I knew) in IE to install a program practically in the background, or to get broader OS control through a browser vulnerability.