Hello Mitch and thanks for these advices.

a) I currently do not really know on which kind of machine I will perform further development, but there a high chances it will use a Cortex-M3 processor. Until then, I will follow your advices and use versatilepb, I will see later for the processor.
 
b) Good job. It works :) The QEMU window's there with the Forth interpreter. I'm somewhat afraid of some warnings tho, here's the output :

johnny@ubuntu:~/openfirmware-mitchbradley/cpu/arm/versatilepb/build$ qemu-system-arm -M versatilepb -m 32 -kernel qemuforth.rom -serial vc
Failed to initialize module: /usr/lib/x86_64-linux-gnu/qemu/block-iscsi.so
Note: only modules from the same build can be loaded.
Failed to initialize module: /usr/lib/x86_64-linux-gnu/qemu/block-curl.so
Note: only modules from the same build can be loaded.
Failed to initialize module: /usr/lib/x86_64-linux-gnu/qemu/block-rbd.so
Note: only modules from the same build can be loaded.
Failed to initialize module: /usr/lib/x86_64-linux-gnu/qemu/block-dmg.so
Note: only modules from the same build can be loaded.
pulseaudio: set_sink_input_volume() failed
pulseaudio: Reason: Invalid argument
pulseaudio: set_sink_input_mute() failed
pulseaudio: Reason: Invalid argument

The ones about audio are probably caused by the lack of audio emulation options, but what about other ones ? Aren't they dangerous ?

Once again, good work, this is all I needed !

c) Thanks for redirecting me to your GitHub version.

d) You are right, I will use qemuforth.rom until I understand some more how OpenFirmware works. I successfully built the rom by replacing the cpu/arm/versatilepb/build/Makefile first line by ROMNAME=qemuforth.

A small error I faced with the subversion's version is still present on your GitHub version when building ARM ROMs:
" I’ve been looking for openfirmware/cpu/x86/Linux/armforth file. As the error states, this file is not present in the cpu/x86/Linux folder. In fact, there is a file named like this, but it has an extension .qemu (the complete file name is armforth.qemu. So I tough about a simple mistake, and renamed it (in fact, I copied it to avoid other errors) into armforth."

Renamed it, and successfully built the ROM.

Thanks for your help Mitch, you've put me in the good direction.
I hope not to have to bother you again, it should be fine from now :)

Regards,

- John