I hate to get into the flame wars anymore, but I have to post this since I live on the web and have worry about coding standards for each browser, etc. A recent study of browser security vulnerabilities shows that FireFox has a significant number of holes. Granted IE has it's share, but consider around 200 for FireFox and 66 for IE, that is a huge gap.
Why is this you say. Well my guess is when you GIVE hackers your blueprints (ie the code) you open the door to easier hacking. Microsoft has a very vested interest in making IE a very strong browser and I personally think they do. I KNOW from my experiences that IE blocks more pop-ups and phishing attacks than FireFox for one.
Also on this topic, since the new version of FireFox was pushed this week this begs the question of what should we expect in IE 8? The answer is, we do not know because the development is kept secret from the world and the hackers. I have to say I like that. Honestly all browsers are really limited to what HTML can offer as far as features that normal people will actually use.