Open Firmware (IEEE-1275 Standard for Boot Firmware: Core Requirements and Practices) is a non-proprietary, platform (CPU and system) independent boot firmware. Similar to a PC's BIOS, Open Firmware is stored in ROM and is the first stored program to be executed upon power-up.
An Open Firmware implementation is based on the Forth programming language, in particular, the FCode dialect (FCode is an ANS Forth compliant dialect that supports compilation of FCode source to bytecode). Apple and Sun are two prominent computer system makers that use implementations of Open Firmware in their systems (Sun's trademark is called OpenBoot). The Open Firmware Working Group's home page is hosted at various places, including Apple and Sun.
Thus, the firmware is implemented in Forth, and stored in the ROM as FCode bytecode. Device drivers that are required during system startup are also implemented similarly. Such drivers usually exist in the expansion ROM of expansion cards that are needed before the operating system has loaded.