Not sure this applies much for the ROM's on microcontroller. They're pretty small and mostly handle very low level details so there doesn't seem much space for meaningful improvement. Even when the firmware is completely open, the original firm often maintains a competitive edge because they have the know how to work with the code base and rest of the chip. Ideally they have internal simulation tools, a broad range of expertise, etc. The firmware often doesn't contain as much "secret sauce", but does contains lots of possible vulnerabilities or bugs for customers.