Perhaps something particular to your machine...out of curiosity I overlaid screenshots of HN on both on Mac and Linux and the text was nearly pixel identical, t

Perhaps something particular to your machine...out of curiosity I overlaid screenshots of HN on both on Mac and Linux and the text was nearly pixel identical, though Firefox seemingly has a taller default paragraph line height than Chrome. There were a handful tiny differences in individual character rendering and spacing, but so minor as to be basically impossible to identify without quickly flipping back and forth between the overlaid layers while zoomed in.