Lower, Mark A (1860) Patronymica Britannica: a dictionary of the family names of the United Kingdom. London: J.R. Smith. Public Domain.
FIRTH
As described in historical dictionaries
Patronymica Britannica (1860)
FIRTH. A parish in Orkney; also a Scottish topographical word, signifying, 1. An restuary or bay ; 2. A sheltered place or enclosure. The etymon in both cases seems to bo the A-Sax. frithian, to protect or shelter.
Origin: Celtic
Region of origin: British Isles
Country of origin: Scotland
Religion of origin: Christian
Language of origin: Gaelic