False. Fiberglass is what air filters are made out of -- it cleans air but doesn't stop it. It can be used to fill a crack if it is then coated with caulking to seal it. Stuffing fiberglass tightly into the space between the window rough and finish frames will not even insulate very well, as compacted fiberglass has little thermal resistance.
Cellulose fiber loose fill is one material that, when packed into a cavity, does block air movement and can serve as an air barrier. It has been successfully used in some cathedral ceilings where ventilation was impossible: the cavity was packed full to prevent air from penetrating, and a good vapour barrier was added to keep out moisture. Ventilation was unnecessary.