FirstLight IV: Diversity in sub-L$_*$ galaxies at cosmic dawn
  Using a large sample of sub-L$_*$ galaxies, with similar UV magnitudes,
M$_{\rm UV}\simeq -19$ at $z\simeq6$, extracted from the FirstLight
simulations, we show the diversity of galaxies at the end of the reionization
epoch. We find a factor $\sim$40 variation in the specific star-formation rate
(sSFR). This drives a $\sim$1 dex range in equivalent width of the
[OIII]$\lambda$5007 line. Variations in nebular metallicity and ionization
parameter within HII regions lead to a scatter in the equivalent widths and
[OIII]/H$\alpha$ line ratio at a fixed sSFR. [OIII]-bright emitters
([OIII]/H$\alpha$>1) have higher ionization parameters and/or higher
metallicities than H$\alpha$-bright ([OIII]/H$\alpha$<1) galaxies. According to
the surface brightness maps in both [OIII] and H$\alpha$, [OIII]-bright
emitters are more compact than H$\alpha$-bright galaxies. H$\alpha$ luminosity
is higher than [OIII] if star formation is distributed over extended regions.
OIII dominates if it is concentrated in compact clumps. In both cases, the
H$\alpha$-emitting gas is significantly more extended than [OIII].
