Don't understand coefficients in MATLAB "butter" low pass result
You are using wrong syntax. You should call freqz function as:

freqz(b,a)


You do not divide any coefficients. In case of non-recursive filters (FIR) you use only coefficients stored in b vector. When you are using the recursive LCCDE (Linear Constant Coefficients Difference Equation) (mostly IIR filters) then you need also the a vector. I suggest you to set a to 1 if you use non-recursive FIR filters.

You should also watch out for numerical problems with transfer function representation as mentioned in final paragraph of this manual entry and this answer. It's always safer to use z-p-k representation.