A narrow inlet of the sea; an estuary:[in place names]:the Moray Firth...
He told her the tales of the sea lochs and the firths that decorated the coast.
It was then stationed off Fife Ness to guide ships approaching the firths of Tay and Forth.
The project would be in addition to the new £89m road crossing already planned 14 miles upstream in the upper firth to take pressure off the existing bridge.
Origin
Middle English (originally Scots), from Old Norse fjǫrthr(see fjord).