Could the redshift be due to photons loosing energy to the cosmic background radiation fireball field?
No because photons do not interact with each other. The photons traveling to us from distant galaxies to show their redshifts, do not interact with the photons in the cosmic background radiation. This property of all photons is not a theory, but a measurable fact. The only way to get around this is to propose that photons do not have zero rest mass, which means they could then interact like high-speed particles. But experiments show that photons have rest masses that are indistinguishable from exactly zero to one part in 10^30 or better!
