Then Firewall would have to be limited by System and Response. You can't say that Firewall doesn't have that normal rating restriction, but make the claim that SnS is "broken" because it supposedly has something the description doesn't say it has. Choose one or the other. Can't have it both ways.
Um....
pg 221 SR 4e main book....
"MATRIX ATTRIBUTES: Firewall, Response, Signal, And System. Two the the Matrix Attributes are based on the hardware in the device, and the other two are software running on that hardware. "
As written, Firewall would be a limited by System and/or Response, just in the same way that your character's Agility is limited by his Willpower or Body. (Which they are NOT!)
here's a way to look at those attributes.
Firewall = Willpower/body* (resists attacks that "affect the mind" IE: sleezing, spoofing, direct damage)
Response = Agility/reaction (how fast the response to stimuli, IE: loading a program,
Signal = Perception/Charisma (how to you interact with your environment, IE: can you access the matrix and nodes from accoss the city, or do you have to rub up against a person to see there profile)
System = Body/Willpower (how much damage you can take, and how you resist that damage)
it is from those four attributes that control of the matrix is given, percieved, used, and combatted. They are not limited by each other, THEY impose limits on everything in the matrix. Much like your character's Attributes limit what you character can do.