The truth is that no browser is completely secure and that every single one has flaws. It comes down to a couple things, in order of importance:

1.) How quickly are flaws patched?
2.) How many serious/critical flaws are there?
3.) How many flaws are there total?
4.) Is the code open source?
5.) What security precautions are there?

In the case of Firefox, it is open source, which means that anyone, including you, can download and review the code for flaws, bugs, and risks. That means that there are thousands of people reviewing the code at any given time whereas Microsoft hires just a handful to do such. Given that, it is much more likely that someone will spot a flaw in Firefox and report it before it is exploited whereas most of IE's flaws are exploited before they are patched. That gives Firefox a significant advantage out of the gate.

Then, consider how long it takes to patch those flaws. Mozilla's turn around time is around 3 days at last check whereas Microsoft relies on monthly patch releases except in certain circumstances, meaning it could be a good 3 weeks before the patch is released.

Third, look at the numbers. Reported flaws in Firefox have increased over the past year, mostly due to its popularity. However, there are still known fewer issues with Firefox than with IE, as well as fewer serious/critical flaws. That word "known" is also important, for Microsoft does its best to keep the reports of flaws to a dull roar, releasing the bare minimum of details. Some speculate that there may be two to three times the number of flaws in IE but that they have simply never been publicized.

Fourth, look at the history of the two browsers. Firefox has a reputation of being more secure and patching flaws faster than Microsoft. While Redmond has shown a renewed interest in Internet Explorer and more of a commitment to secure it they still have years of neglect and security flaws hovering over them. They have yet to prove that they can keep the browser up-to-date and minimize the flaws, something it may take a few years to convince the general public.

Finally, look at what abilities you, the user, have to keep yourself safe. Firefox has thousands of extensions available, some of which (such as SiteAdvisor, NoScript, and NetcraftToolbar) can help make your browser more secure than it already is. IE, by contrast, has just a few.

Thus, I would still consider Firefox more secure than Internet Explorer.

Aside from that, IE7 has numerous non-security related issues to deal with, including installation/uninstallation woes, incompatibilities with various websites and security software, and scrolling problems when used with some Logitech mice.

In short, IE still has a long way to travel before it will have a legitimate chance of winning back the public's trust and drive up its market share again.

John