The welcome sign in Firth Idaho. The sign advertises the Little Buckaroo Rodeo, which is a rodeo for children. – Sociotard, CC BY 3.0 – Wikimedia Commons Firth is located in Bingham County. About 517 people live here (as of 2020). Between 2010 and 2020, the population grew from 477 to 517. The region is at 1393 meters above sea level. The area covers about 1.5 square kilometers. There are about 31 streets in Firth. The postal codes are 83236. The dialing code is 208. Firth is city in Bingham County, Idaho, United States. Among other things, Firth has restaurants and supermarkets.