Antiferromagnets are promising for magneto-optical light control that could be performed at THz frequencies via excitation of the quasi-antiferromagnetic spin modes. However, most of the antiferromagnetic crystals possess optical anisotropy that is usually treated as an unfavorable condition for the magneto-optical measurements: optical anisotropy is known to diminish the Faraday rotation with respect to the case of the isotropic medium. Here we show that the situation could be quite opposite: a phenomenon of birefringence mediated enhancement of the magneto-optical activity appears if the orientation of the incident light linear polarization is chosen properly. The present study relies on the experimental, analytical, and numerical studies of iron borate FeBO3 crystals. We demonstrate a significant increase of the magneto-optical activity by more than 10 times for 70 • angle between light polarization and incidence plane instead of commonly-used p-or s-polarizations. It provides a unique sensitivity to the in-plane magnetization of FeBO3 that is crucial for the pumpprobe studies, magneto-optical microscopy, and others. The most important practical application of the observed phenomenon is the light modulation with up to 100% efficiency at THz frequencies. The approach is applicable to other types of birefringent crystals with the magneto-optical response.