If you all have been developing software for a while then you will know that in this field there is always security lapse and bugs fixes. This is part and parcel of software development life cycle.
The fact that if a old software is used for many years without problem does not indicate it is perfect. It just runs fine with most typical scenarios. Once you are into developing software you will know what I mean.
So be it FF or IE, as long as it is software, there will be bugs and security lapse. The debate is more on how frequent it is faulty or leaky or buggy among competitive products reported by users
