The Packard-Bell PB440 (1963) was microprogrammable:
https://bitsavers.org/pdf/packardBell/PB-440/SP-149A_PB-440_microprogrammin…
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