In OS 9, the bootup screen lives as a .cpp file (C++ file) embedded into the file named "Mac OS ROM", at the root of the System Folder. (I haven't been able to locate it on OS X.) So it's not in the firmware, it's in the software. It was moved there after Apple cleaned up the PPC machine ROM and moved a lot of stuff into that Mac OS ROM file, to make the data easier to modify it later.