IE seems to have more security vulnerabilities, because more people actively go out and search for them. Without a doubt, IE is the single biggest browser used all over the world (>90%)... so its only natural that more security flaws are found within the browser. I'm sure if firefox was used by the same proportion of people, a lot more security flaws would probably be found.
That said, I'm using firefox right now, because less people are looking for security flaws within it
-Rich