  In this article we prove modular and norm P\'olya-Szeg\"o inequalities in
general fractional Orlicz-Sobolev spaces by using the polarization technique.
We introduce a general framework which includes the different definitions of
theses spaces in the literature, and we establish some of its basic properties
such as the density of smooth functions. As a corollary we prove a
Rayleigh-Faber-Krahn type inequality for Dirichlet eigenvalues under nonlocal
nonstandard growth operators.
