ΔE 16.4, loose: different enough that you should pick on purpose. Orange Rust is noticeably lighter, more muted, warmer than Firethorn Brown.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Orange Rust is noticeably lighter, more muted, warmer than Firethorn Brown.
The same difference applies the other way round: Firethorn Brown is noticeably darker, more saturated, 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 Firethorn Brown on the colour wheel See Orange Rust on the colour wheel
All matches: Firethorn Brown equivalents · Orange Rust equivalents