We develop a fully kinetic linear theory of the drift mirror (DM) instability accounting for arbitrary particle velocity distribution functions including nonzero electron temperature effects and plasma pressure anisotropy. In the quasi-hydrodynamic limiting case the theory reproduces the results obtained for the ion mirror instability. However, for the very low frequency electron DM modes which can develop in a nonuniform plasma of nonzero electron temperature, Te ≠ 0, such an equivalence does not exist. We refer to these modes as slow drift mirror (SDM) modes in order to distinguish them from the conventional ion-DM mode. Two new instabilities, one hydrodynamic and one kinetic, that lead to the growth of SDM modes are found in the fully kinetic regime. The first instability develops for values of the ion anisotropy lower than required for the classical ion-DM instability. The second instability occurs when the conditions for the ion-DM instability are satisfied as well. The frequency of the SDM mode is less than the frequency of the ion-DM mode, ω ≪ ωDM ~ ωn, where ωn is the density gradient-drift frequency of the ions. However, when the electron temperature is of the order of the parallel ion temperature, the SDM instability growth rate may become comparable or even higher than that of the ion-DM instability. The free energy necessary for these new instabilities is taken from two sources. The main source is the energy stored in the ion pressure anisotropy. The second source is the electron pressure gradient which builds up in a plasma of nonzero electron temperature.