Firth is a word in the Scots and English languages used to denote various coastal waters in Scotland and England.In mainland Scotland it is used to describe a large sea bay, or even a strait. In the Northern Isles it more usually refers to a smaller inlet.owever, some firths are exceptions. The Cromarty Firth on the east coast of Scotland, for example, resembles a large loch with only a relatively small outlet to the sea and the Solway Firth and the Moray Firth are more like extremely large bays.