As far as I understand

Safari is based on open source Konqeror like OS X is based on BSD Unix.

But to the point, certainly MS has had to react to Firefox. Half heartedly, perhaps, because there is no "business money" in it and perhaps because of the conflict between keeping the non standard Microsoft standard as a lock in and the ugly demand for a new IE with a better non Microsoft standard.

Somehow I think Firefox should, perhaps, also mention the "O" word when speaking about Firefox.