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The threads are stripped out of the plug hole. It will have to be drilled out, rethreaded, and a HELICOIL installed. NOT recommended for the average backyard mechanic. Have a reputable shop perform this work.
The issue seems to be related to the spark plug threads. The spark plug is either not tightened down or the threads are stripped, and during the compression stroke when pressure is applied inside the combustion chamber, the pressure leaks thru the spark plug threads and blows off the spark plug boot. If the spark plug is not tightened and the threads are ok, then tightening the plug will resolve this issue. If the spark plug threads are damaged, then the threads will need to be repaired, and will require the cylinder head to be removed to repair the stripped threads. Taking the cylinder head off requires the engine's top end to be ripped apart and re-assembled....big job.
The spark plug blew out because it was not installed correctly and was loose. Install a new plug and do it very carefully as there may be carbon in the treads the plug screws into. Screw it in until you feel it getting tight and then turn it back. Keep turning in and out until it fully seats. Always follow the vehicle manufacturer torque recommendations, but as a general rule 14 mm plugs with a gasket style seat should be tightened to 26 to 30 ft.lbs. in cast iron heads, but only 18 to 22 ft.lbs. in aluminum heads. Likewise, 18 mm plugs with gasket style seats should be tightened to 32 to 38 ft.lbs. in cast iron heads but only 28 to 34 ft.lbs. in aluminum heads. For taper seat spark plugs, 14 mm plugs should be tightened to 7 to 15 ft.lbs. in both cast iron and aluminum, while 18 mm taper seat plugs should be tightened to 15 to 20 ft.lbs. in both types of heads.
Ford had trouble with spark plugs blowing out on some of their motors. It happened most often on v-10 triton motors though. Ford and some aftermarket companies make a kit to repair this without removing the head. I believe it comes with drill bits, a tap and a helicoil. A call to the ford dealer or a google search for a kit should get you what you need.