ΔE 16.2, loose: different enough that you should pick on purpose. Grunge Brown is a little lighter, much more muted than Firethorn Brown.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Grunge Brown is a little lighter, much more muted than Firethorn Brown.
The same difference applies the other way round: Firethorn Brown is a little darker, much more saturated.
ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.
See Firethorn Brown on the colour wheel See Grunge Brown on the colour wheel
All matches: Firethorn Brown equivalents · Grunge Brown equivalents