Firth is a city in Bingham County, Idaho, United States. The population was 477 at the 2010 census.
Firth is located at 43°18′20″N 112°11′02″W (43.305519, -112.183938).
According to the United States Census Bureau, the city has a total area of 0.54 square miles (1.40 km2), all of it land.
Firth is located on the eastern side of the Snake River, facing the Blackfoot Mountains.
Firth began as a Swedish settlement in 1885. It was named for Lorenzo J. Firth, an English emigrant, who gave land for the railroad section house and water tank; the railroad named the station for him in 1903. The post office was established in 1905.
At the 2010 census there were 477 people in 168 households, including 121 families, in the city. The population density was 883.3 inhabitants per square mile (341.0/km2). There were 173 housing units at an average density of 320.4 per square mile (123.7/km2). The racial makup of the city was 79.5% White, 2.1% Native American, 0.4% Pacific Islander, 14.5% from other races, and 3.6% from two or more races. Hispanic or Latino of any race were 25.2%.
Of the 168 households 42.3% had children under the age of 18 living with them, 57.1% were married couples living together, 10.7% had a female householder with no husband present, 4.2% had a male householder with no wife present, and 28.0% were non-families.