The new leaves will rob fertilizer from the older leaves so the older leaves turn yellow then brown and drop off. Older leaves on ficus tree in greenhouse have brown spotches on back and leaves eventually fall off. If you can remove brown spots it is scale. Generally ficus looses leaves when it is moved from one location to another and there is a change from more to less light. Ficus needs bright light and plenty of water.
Older plants lose old leaves as new ones come on.
Ficus also have a disease called Oedema which is a nonparasitic disease. When the relationship of the plant with water are abnormal as under conditions of exceptional humidity, small masses of tissue may expand and break out on the surface causing a watery swelling or gall. Frequently the exposed surface becomes rusty in color so that the lesion may be mistaken for a rust caused by a fungus. Any plant will help clean the air. The bigger the leaves the more cleaning effect they have.