> etymonline points out that the form with a better-in-some-sense claim to be "correct" is dwarrows, not "dwarfs". But that term was uncommon enough that it completely died out.

This form appears in LOTR in "Dwarrowdelf", given as the translation of the Dwarvish name "Khazad-dûm" of Moria. https://en.wikipedia.org/wiki/Moria_(Middle-earth)