ΔE 13.7, fair: the same colour family, but not a straight swap. Firethorn Brown is a little lighter, more saturated, warmer than Mephiston Red.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 14). Firethorn Brown is a little lighter, more saturated, warmer than Mephiston Red.
The same difference applies the other way round: Mephiston Red is a little darker, more muted, cooler.
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 Mephiston Red on the colour wheel See Firethorn Brown on the colour wheel
All matches: Mephiston Red equivalents · Firethorn Brown equivalents