  Little is known about the mass-metallicity relation (MZR) in galaxies at
cosmic dawn. Studying the first appearance of the MZR is one of the keys to
understanding the formation and evolution of the first galaxies. In order to
lay the groundwork for upcoming observational campaigns, we analyze 290
galaxies in halos spanning Mh = 10^9-10^11 Msun selected from the FirstLight
(FL) cosmological zoom simulations to predict the MZR at z=5-8. Over this
interval, the metallicity of FL galaxies with stellar mass Ms= 10^8 Msun
declines by < 0.2 dex. This contrasts with the observed tendency for
metallicities to increase at lower redshifts, and reflects weakly-evolving or
even increasing gas fractions. We assess the use of the R3 strong-line
diagnostic as a metallicity indicator, finding that it is informative for
12+log(O/H) < 8 but saturates to R3=3 at higher metallicities owing to a
cancellation between enrichment and spectral softening. Nonetheless, campaigns
with JWST should be able to detect a clear trend between R3 and stellar mass
for Ms > 10^7.5 Msun. We caution that, at fixed metallicity, galaxies with
higher specific star formation show higher R3 owing to their more intense
radiation fields, indicating a potential for selection biases.
