An oft repeated myth, but it doesn't get any more true the more it's repeated.
Just because people aren't writing code to attack Firefox, doesn't mean it is invulnerable.
Of course not. Firefox has it's share of bugs. But they will less nasty because it's not imbedded into the OS, it's built by a group of developers that believe that standards are more important than marketshare and that reliability and usability are more important that market domination.
I wasn't repeating anything I have heard or read anywhere else. This was my own observation. As I stated before, I am neutral. I will use any OS that is good for me. But I still believe and always will that there is no 'invulnerable' software, and Firefox is just below the radar at the present. If I were writing destructive software, I would target the largest number of platforms with the smallest amount of effort.
I understand your point of FF being a separate program and not imbedded.