Upon their gods; either,
1. Their princes and rulers, who are sometimes called gods in Scripture; and so this is added by way of amplification, God slew their first-born; not only of the meaner sort, but even of their king and princes. Or,
2. Their false gods, to wit, those beasts which the brutish Egyptians worshipped as gods, which were killed with the rest, for the first-born both of men and beasts were then killed, Exodus 13:5. See Poole on "Exodus 12:12"; See Poole on "Exodus 18:11".