If I had a nickel for every time someone has asked me to explain what a firth is, I would have almost five cents by now.
Mostly it's people who live closer to Dayton than Dundee who ask such questions, or the followers of pasture pool more accustomed to Augusta than St. Andrews. Nevertheless, even such as these deserve to go to their graves no longer furtively firth-ignorant. Fer sure.
Afore one speaks of firths, forsooth, one must firth speak of Forth. First and foremost, friends, the Forth is a river. A fine, fine, wondrous river of fabulous and fantastic beauty. Yet everything must eventually come to an end, and so it is with beautiful rivers as well. These wide river-ends the English call estuaries, but those finer folk further in the bracing north call them firths. Not just because the language is more refined in the north, mind you, but... well, because that is what they ARE, you see. Firths.
And, aflight or afoot, near or afar, you'll find no finer firth than the Firth of Forth. Frankly. Yet here we must pause to ensure our quaint colonial friends are not led far afield in their frantic fathomings, for it must be explained - firmly - that as fine as it fairly is, this firth, this fantastical Firth of Forth, it is actually a fjord.
I am at a loss of words, at least F-words, to continue.