  We investigate thin and thick stellar disc formation in Milky-Way-mass
galaxies using twelve FIRE-2 cosmological zoom-in simulations. All simulated
galaxies experience an early period of bursty star formation that transitions
to a late-time steady phase of near-constant star formation. Stars formed
during the late-time steady phase have more circular orbits and thin-disc-like
morphology at $z=0$, whilst stars born during the bursty phase have more radial
orbits and thick-disc structure. The median age of thick-disc stars at $z=0$
correlates strongly with this transition time. We also find that galaxies with
an earlier transition from bursty to steady star formation have a higher
thin-disc fractions at $z=0$. Three of our systems have minor mergers with
LMC-size satellites during the thin-disc phase. These mergers trigger short
starbursts but do not destroy the thin disc nor alter broad trends between the
star formation transition time and thin/thick disc properties. If our
simulations are representative of the Universe, then stellar archaeological
studies of the Milky Way (or M31) provide a window into past star-formation
modes in the Galaxy. Current age estimates of the Galactic thick disc would
suggest that the Milky Way transitioned from bursty to steady phase $\sim$6.5
Gyr ago; prior to that time the Milky Way likely lacked a recognisable thin
disc.
