Hello
The latest fast Firefox v57.0.3
/tmp/tcloop/firefox/usr/local/firefox/firefox: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), dynamically linked, interpreter /lib/ld-linux.so.2, for GNU/Linux 2.6.18, BuildID[sha1]=60d514e7a6d8f1901cf6848d5f2c33d2692b10cd, stripped
AIUI Firefox is compatible with 386 and up, but I don't have anything less than a 686 machine to test on
Given Juanito's previous input i'm wondering now if file is just reporting the host compile machine arch. (though i doubt a 386 was used to compile firefox on)