There are little plastic rollers on the window mechanism that go into the tracks. They are held on by a little metal clip. When they break, the window falls in. They are cheap, go to
www.texasmustang.com or some other Mustang parts store. While you are at it, find out how many you need, replace them all, your windows will love you for it. They are one size fits all, I think it takes 8, but I can't remember, probably less than $20. total