The Forth programming language is not mainstream these days. But from firmware perspective: OpenFirmware was written in the Forth programming language. And now there’s a Forth compiler that targets UEFI applications. Jonas Hvid has written Jonasforth, a Forth interpreter that is a UEFI application, written in Flat Assembler (FASM). So it works on x64-based Intel UEFI/AMD (but not ARM or RISC-V or non-x64) systems.
Forth aside, this is a FASM-based UEFI application, most FASM-based UEFI apps are ‘hello world’ level, so this gives a more substantial application to study.