\[ab^{3} - b^{3} + ab^{2} - b^{2} =\]
\[= b^{3}(a - 1) + b^{2}(a - 1) =\]
\[= (a - 1)\left( b^{3} + b^{2} \right) =\]
\[= b^{2}(a - 1)(b + 1).\]