The Packard-Bell PB440 (1963) was microprogrammable: https://bitsavers.org/pdf/packardBell/PB-440/SP-149A_PB-440_microprogramming_May63.pdf
Various earlier systems used microprogram-like techniques, but I don't know how many were earlier than PB in their use of the word. Was the PB-250's floating point microprogrammed? --Chuck
