  Models of star formation in the early universe depend on the details of
accretion, fragmentation and radiative feedback. Different simulations predict
different initial mass functions of the first stars, ranging from predominantly
low mass (0.1-10 Msol), to massive (10-100 Msol), or even supermassive
(100-1000 Msol). The mass distribution of the first stars should lead to unique
chemical imprints on the low-mass second and later generation metal-poor stars
still in existence. The chemical composition of SMSS0313-6708, which has the
lowest abundances of Ca and Fe of any star known, indicates it was enriched by
a single massive supernova. However, even weak spectral lines may be affected
by strong 3D and NLTE effects in metal-poor stars. If these effects are ignored
or treated incorrectly, errors in the inferred abundances may significantly
bias the inferred properties of the polluting supernovae. We redetermine the
chemical composition of SMSS0313-6708 using 3D NLTE radiative transfer to
obtain accurate abundances for Li, Na, Mg, Al, Ca and Fe. The model atoms
employ realistic collisional rates, with no calibrated free parameters. We find
significantly higher abundances in 3D NLTE than 1D LTE by 0.8 dex for Fe, and
0.5 dex for Mg, Al and Ca, while Li and Na are unaffected to within 0.03 dex.
In particular, our upper limit for [Fe/H] is now a factor ten larger, at [Fe/H]
< -6.53 (3 sigma), than previous estimates based on <3D> NLTE (i.e. using
averaged 3D models). This higher estimate is due to a conservative upper limit
estimation, updated NLTE data, and 3D-<3D> NLTE differences, all of which lead
to a higher abundance determination. We find that the revised chemical
composition of SMSS0313-6708 matches supernova yields for massive progenitors
of 20-60 Msol exploding with low energies (1-2 x 10^51 erg), as well as
progenitors of 10 Msol with very low explosion energies (< 10^51 erg).
