The mount on the frame is broken. You'd need to take the seat cover and foam off and then fix it. But the frame will be bent still and probably never be right again... JRichker has a post on how to correct the issue.
Mine leans towards the center of the car. I'll just replace my exisiting seats with newer ones one day.
mine leans towards the shifter a little....i try, like when im on the highway a while, to drive with my right hand instead of left which puts more pressure the opposite way so that they wont get too much more worse.
The seat problems come in two categories:
1.) The floor pan that holds the front seat mounts cracks and allows the stud that secures the seat to pull out. I recommend that you take two L shaped pieces of 16 gauge steel and overlap the damage. Cut and fit one "L", drill the bolt hole and the weld a bolt in the hole that is for the missing stud. Then weld it in place. The repeat the cut and fit of the next "L" and weld it in place.
2.) The seat back frame cracks and splits. I riveted mine with some solid monel rivets. You can weld the frame if you don't a have a rivet gun and steel rivets. Removing the head rest is the difficult part. See http://forums.stangnet.com/showthread.php?t=478804&highlight=seat and you'll find pictures and instructions. Once the head rest is off, unzip the upholstery and slide it off the metal frame.
When you take the upholstery off the seat frame you will see little tears in the metal or maybe a few bigger ones or sometimes the pop welds break. I have seen all 3. We usually just stick a long rod through the seat frame and twist it back as close to normal as you can and then weld it back up.