Astrophysical BBH primary mass distribution of all the local (z ≤ 1) BBH mergers sampled in FIREbox. Different colours correspond to different massive star cluster formation models. We apply a re-weighting factor corresponding to the halo mass of the host galaxy (see Sect. 2.1) to each BBH merger. All the distributions are normalised to the predicted value for the BBH merger rate density at z = 0. The blue curve shows the differential merger rate as a function of primary mass from GWTC-3 analyses (Abbott et al. 2023a), with the shaded region showing the 90% credible interval.