estuary vs firth

estuary and firth both are nouns.

Word	Noun	Adjective	Verb	Adverb
estuary	Yes	No	No	No
firth	Yes	No	No	No

As nouns, firth is a hyponym of estuary; that is, firth is a word with a more specific, narrower meaning than estuary:

estuary: the wide part of a river where it nears the sea; fresh and salt water mix
firth: a long narrow estuary (especially in Scotland)

estuary (noun)	firth (noun)
the wide part of a river where it nears the sea; fresh and salt water mix	a long narrow estuary (especially in Scotland)