The Firth of Forth is the estuary or firth of Scotland 's River Forth . It flows into the North Sea , between Fife to the north , and West Lothian , the City of Edinburgh and East Lothian to the south . The Firth of Forth is the estuary or firth of Scotland 's River Forth , where it flows into the North Sea , between Fife to the north , and West Lothian , the City of Edinburgh and East Lothian to the south .