As a hacker, I can say with 100% certainty that Firefox is a better browser in terms of security than IE. IE's severely flawed implementation of HTML makes it so that just about every XSS (Cross Site Scripting) attack in the book works on it.
I can do IE-Compatible XSSes via tables, images, xml tags, bgsound tags, object tags, frame tags, iframe tags, and on and on. Not nearly as many of those will work in Firefox. In fact ALL of the XSSes I have done have worked in IE. However, only a handful have worked in Firefox.
About the patching of other security flaws that have been discovered, flaws discovered in Firefox (of which far fewer have been discovered than in IE, and the criticality of them has been lower) have been patched in short order. However, Microsoft has taken months to patch security flaws in IE before.
As far as features go, Firefox has a good number of them built in. Even if there is less than IE, Firefox has a robust plug-in interface; a plug-in can be written to do most anything in Firefox.
I even have a cookie editor for Firefox so I can effortlessly forge cookies. Forged cookies can be used to get unauthorized access to website features, and even entire accounts, so the cookie editor plug-in is really handy. There are MANY plug-ins for Firefox to add more features and functionality to the browser. And more are being developed every day.
These reasons are why Firefox is the browser of choice for hackers. It is more secure and more extensible.
By the way, while previewing the first version of my post, I discovered that this comments page may be vulnerable to XSS. I will do a test post to make sure (Don't worry, it will just create a simple alert() box if successful.)