The interstellar medium (ISM) of the Milky Way is found to be multiphased, composed of cold neutral medium (CNM, T=100K, and density of n=30cm^-3) which coexists with the warm neutral medium (WNM, T=1e4 K, and n=0.3 cm^-3). The intermediate region is thermally unstable. The multiphase structure of the ISM, and the thermal instability region, may be responsible for regulating star-formation in our Galaxy, and in other normal galaxies. I will present recent results (Bialy and Sternberg, in prep), where we show that at low metallicities, as those expected in the first galaxies in the early Universe, the multiphase phenomena totally disappears, and the gas cannot cool below 500 K. These findings may have important consequences for the active modes of star-formation in the first galaxies, some 13 Gyrs ago.