Firefox not only has more vulnerabilities per month than Internet Explorer, but it is now surpassing Internet Explorer for the number of exploits available for public download in recent months
My least favorite advertising is the "hey people don't put viruses on us!" type. Because what that really means is that no one cares about their software enough to hack it... so what they really are saying is "This is great because no one uses it", which really isn't an argument at all.
Personally I use Firefox, my two reasons are tabbed browsing, and a "blog this" option on a right click. If IE were to implement this I might stat to use them again (IE uses less system resources than Firefox).