I'm a bit confused about the question. The PowerBook 1400 was an Old World (and in fact a NuBus Old World) Macintosh. It contained a Masked programmed ROM, not a Flash based ROM. This means the ROM can't be updated. Also, because it's an Old World Mac, there's no Toolbox ROM on the disk. So, there are no firmware updates.
ROM based Macs also didn't need firmware updates, since Mac OS itself effectively updated the ROM by patching the API entry points. On a 68K Mac this was done by patching the 0xA000 and 0xF000 Trap vectors with new code loaded from disk.