What do you mean that isn't definite? I'm very in favor of open firmware, but there have been multiple examples of clones popping up whenever firmware is public and open source.
Open-source firmware wouldn't exactly be the same thing as public domain firmware. And even if a product's firmware isn't getting the benefit of copyright protection, the product as a whole can still be protected by patents and trademarks.
If they construct a new device but reuse the firmware, it's sort is the point of having open firmware.