When only one tire is affected it is due to it being tipped in on top. That is called "camber" and is one of the three primary alignment angles.
Ford has way more steering and suspension part failures than anyone else so the first thing is to have it inspected at a tire and alignment shop. The ball joints are the most common parts that wear. Tie rod ends cause a lot of problems too but that will affect "total toe", another primary alignment angle, and that will affect both front tires nearly equally. Incorrect toe also usually causes a feather-edge wear pattern across the tread, but you won't see that if the tread is almost gone.
Worn control arm bushings can also let the tire tip but you will typically hear some kind of clunking. If no worn parts are found, have the vehicle aligned. On the printout they give you, you should see the "before" reading for left front camber is a negative number meaning tipped in on top. The range of acceptable specs will be listed too and you should see the "after" number is closer to the middle of that range.