Let $R$ be (not necessarily commutative) ring and $S$ a simple right $R$module. Let $f\in Ann(S)$ be normalizng and a nonzero divisor. Is it always true that $$ pdim_{R}(S)=pdim_{R/(f)}(S)+1? $$

With the definition of normalizing you give, it is not always the case that the projective dimension of $S$ as an $R$module equals $1$ more than the projective dimension of $S$ as an $R/\langle f \rangle$module. Let $R$ be $\mathbb{Z}$, let $f$ be $p^2$ for some prime $p$, and let $S$ be $\mathbb{Z}/p\mathbb{Z}$. The projective dimension of $S$ as an $R$module is $1$, but the projective dimension of $S$ as an $R/\langle f \rangle$module is infinite. 

