Firth, ID has a population of 437 and is the 20,378th largest city in the United States. The population density is 762 per sq mi which is 3896% higher than the Idaho average and 742% higher than the national average. The median age in Firth is 37 which is approximately 4% higher than the Idaho average of 36. In Firth, 75% of the population over 15 years of age are married, 85% speak English and 15% speak Spanish. 68% of Firth residents were born in Idaho, 24% were born out of state, 3% were born outside of the United States and 4% were foreign born.