Are most extension cord fires caused by manufacturing defects?

No, most extension cord fires are caused by misuse, not manufacturing defects. The big three causes are overloading (too much current through a thin cord), damaged insulation (cuts or cracks), and pinching under furniture or rugs. Inspect cords by feeling for cuts, kinks, or soft spots, and check for bent prongs or loose plugs. If damaged, throw it away—cords are cheap, fires are not.