The Firth of Forth is an estuary (or firth) of the River Forth in Scotland. I wonder if that was on the mind of the writer for Yahoo! News:
Probably not. I doubt that anyone at Yahoo! News knows what a firth is. The rest of us know that, at least in this headline, it’s wrong. The writer may have meant forth, which he or she confused with fourth. (It’s been known to happen.) The first sentence under the headline isn’t any help. That looks like a typo for filth.
Any guesses as to what those two “words” are supposed to be?