There is nothing wrong with either browser from a security point of view. I use both depending on my mood. If IE 7 64 bit worked with IE 7 pro (as of this post), I would give the edge to IE 7 (although FF 3 is a very nice improvement over FF 2).
When FF came out, the taunt was that it was more secure but as more users adopted it the “bad guys” have had reason to target it. FF does not natively support Active X, the primary target of IE hacks, but that has not stopped the “bad guys” from using alternate methods of attack. Both browsers have susceptibilities.
At this point in time, it really comes down to preference. As I mentioned, I use both and I think they are equally qualified from both a performance and security standpoint.
Being a wise ‘net surfer regardless of what you choose is the bottom line.
~Wile