  Recent observations indicate that galactic outflows are ubiquitous in high
redshift galaxies, including normal star forming galaxies, quasar hosts, and
dusty star forming galaxies (DSFGs). However, the impact of outflows on the
evolution of their hosts is still an open question. Here, we analyse the star
formation histories (SFH) and galactic outflow properties of galaxies in
massive haloes ($10^{12}M_{\odot}<M_{\rm vir} <5\times 10^{12}M_{\odot}$) at
$z\gtrsim5.5$ in three zoom-in cosmological simulations from the MassiveFIRE
suite, as part of the Feedback In Realistic Environments (FIRE) project. The
simulations were run with the FIRE-2 model, which does not include feedback
from active galactic nuclei (AGN). The simulated galaxies resemble $z>4$ DSFGs,
with SFRs of $\sim 1000\ M_{\odot}\rm yr^{-1}$ and molecular gas masses of
$M_{\rm mol}\sim 10^{10}\ M_{\odot}$. However, the simulated galaxies are
characterised by higher circular velocities than those observed in high-z
DSFGs. The mass loading factors from stellar feedback are of the order of $\sim
0.1$, implying that stellar feedback is inefficient in driving galactic
outflows and gas is consumed by star formation on much shorter time-scales than
it is expelled from the interstellar medium (ISM). We also find that stellar
feedback is highly inefficient in self-regulating star formation in this
regime, with an average integrated star formation efficiency (SFE) per
dynamical time of $30\%$. Finally, compared to FIRE-2 galaxies hosted in
similarly massive haloes at lower redshift, we find lower mass loading factors
and higher SFEs in the high redshift sample. We argue that both effects
originate from the higher total and gas surface densities that characterise
high$-z$ massive systems.

Astrophysics of Galaxies