Apple used to use it in the PowerPC days as did Sun - https://en.wikipedia.org/wiki/Open_Firmware http://www.brendangregg.com/Sun/obp.html

Apple used it when they were still PowerPC-based. They sadly dropped it with the move to Intel.

You used to be able to boot up with Command+O+F and get a Forth prompt. (Although I have a hazy recollection that on some models they didn't include a screen terminal, so you couldn't interact with it...)