The noun FIRTH has 2 senses: 1. English linguist who contributed to linguistic semantics and to prosodic phonology and who was noted for his insistence on studying both sound and meaning in context (1890-1960). 2. a long narrow estuary (especially in Scotland). Familiarity information: FIRTH used as a noun is rare.irth of Clyde (a firth on the southwestern coast of Scotland emptying into the North Channel). Firth of Forth (a large firth on the east coast of Scotland and the estuary of the Forth River; location of Edinburgh). Moray Firth (an inlet of the North Sea on the northeast coast of Scotland).