Federal Firefighters Fairness Act of 2021 This bill provides federal workers' compensation to firefighters who contract certain illnesses as a result of their service. Specifically, the bill provides that (1) heart disease, lung disease, and specified cancers of federal employees employed in fire protection activities for at least 5 years are presumed to be proximately caused by such employment if the employee is diagnosed with the disease within 10 years of employment; and (2) the disability or death of the employee due to such disease is presumed to result from personal injury sustained in the performance of duty. These presumptions also apply to fire protection employees (regardless of the length of employment) who contract any communicable disease at the center of a designated pandemic or any chronic infectious disease that the Department of Labor determines is related to job-related hazards. An employee in fire protection activities is a firefighter, paramedic, emergency medical technician, rescue worker, ambulance personnel, or hazardous material worker, who (1) is trained in fire suppression; (2) has the legal authority and responsibility to engage in fire suppression; (3) is engaged in the prevention, control, and extinguishment of fires or response to emergency situations where life, property, or the environment is at risk; and (4) performs such activities as a primary responsibility.