The average grade salary at Firth District was $58,813 , which is 5 percent lower than the average salary for this job in Idaho, and 3 percent lower than the national average for this job . Grade salaries at Firth District typically range from $52,050 to $64,972 (25th to 75th percentile). The highest reported grade salary was $67,024. There are 17 employees at Firth District with the job title grade.