There’s a division of Microsoft known as the "Trustworthy Computing Group". That sounds nice. The head of the aforementioned group is Jeff Jones. He says that Internet Explorer has historically needed fewer security fixes than the competition and, as such, IE is the more secure browser.
Jones says that Mozilla has had to fix almost 200 Firefox vulnerabilities since 2004 whereas Microsoft has only needed to fix 87 Internet Explorer problems. He also points out that Microsoft still supports IE 5 while Mozilla only supports Firefox 2 and up.
Couple things to consider here. Firefox has historically been a much more customizable, extendable browser than Internet Explorer. More features invite more possible problems along the way. Also, Firefox seems to get updated more than IE and the changes seem to be more drastic. Internet Explorer hadn’t changed much until IE7 so there wouldn’t have been as many holes to plug between versions four, five, and six.
It’s kind of like comparing landline telephone service to cell phones and saying that landlines are more reliable. Sure they are, but you can’t really do much with them except talk. Cell phones let you leave the house, listen to music, look up information, and all that good stuff. It’s a trade off. Plus, landlines have been around a lot longer than cell phones so there’s been more time to work out the kinks.
Microsoft says Internet Explorer more secure than Firefox [heise Security]