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The continuous growth of the cloning of electronic devices poses a severe threat to our critical infrastructure that uses the Internet, as cloned devices can transmit secret information and cause security concerns. Cloned devices can also be unreliable as they may be manufactured with inferior quality materials, and they may have many defects as they may not be tested properly. It is thus extremely important to protect these electronic devices from cloning. An efficient way to prevent a device being cloned is to prevent the firmware from being copied because, without the proper firmware, the device will not function like the original. In this paper, we present a novel firmware obfuscation method without encrypting the entire memory. The firmware is obfuscated by swapping a subset of instructions. The instructions to be swapped are specifically chosen so that an attacker cannot discover their location. During operation, the hardware reconstructs the original program using a PUF-generated identifier (ID) and a small memory that stores the swapped instructions. An adversary cannot make a program work completely without knowing which instructions have been swapped, as the program will execute in the wrong sequence and produce the incorrect result. Our proposed solution requires only a small overhead to reconstruct the firmware, making it practical for devices with strict resource constraints. This solution also allows remote updates of new obfuscated firmware without any modification and is practical for the rising trend of ubiquitous computing.