In the FirstLight project , we have used \sim 300 cosmological , zoom-in simulations to determine the star-formation histories of distinct first galaxies with stellar masses between M _ { * } = 10 ^ { 6 } and 3 \times 10 ^ { 9 } M _ { \odot } during cosmic dawn ( z = 5 - 15 ) .
The evolution of the star formation rate ( SFR ) in each galaxy is complex and diverse , characterized by bursts of star formation .
Overall , first galaxies spend 70 % of their time in SF bursts .
A sample of 1000 of these bursts indicates that the typical burst at z \simeq 6 has a specific SFR ( sSFR ) maximum of 5 - 15 { Gyr } ^ { -1 } with an effective width of \sim 100 Myr , one tenth of the age of the Universe at that redshift .
A quarter of the bursts populate a tail with very high sSFR maxima of 20 - 30 { Gyr } ^ { -1 } and significantly shorter timescales of \sim 40-80 Myr .
This diversity of bursts sets the mean and the mass-dependent scatter of the star-forming main sequence .
This scatter is driven by a population of low-mass , M _ { * } \leq 10 ^ { 8 } M _ { \odot } , quiescent galaxies .
The mean sSFR and the burst maximum at fixed mass increase with redshift , with the later always being a factor \sim 2 higher than the former .
This implies sSFR maxima of \sim 20 - 60 { Gyr } ^ { -1 } at z = 9 - 10 .
The SFR histories are publicly available at the FirstLight website .