DORNOCH Firth, an arm of the sea on the E. coast of Scotland, serving as the boundary between the counties of Ross and Sutherland. It is about 15 miles broad at its mouth, and here has more the character of a bay than of a firth. It gradually becomes narrower, till, about three miles west from the town of Dornoch, its breadth decreases to two miles. Above this point it becomes much broader, forming an inner harbour or bay.