Firefox does not allow a website to install software on your computer in the background without your knowledge... period. For this simple reason alone, it will never be as insecure and as much of a doorway into the rest of your computer as IE.
Average Joes approve any installation anyways. I've seen it happen myself. A website would ask if he/she wants to install this piece of software and users would just click Yes no matter what. Just to get rid of the window, without even reading what it says. So if Average Joes will start using FF, they will confirm any kind of downloads too.