Tire Sidewall problems of the 2001 Nissan Xterra

I purchased 4 firestone destination le tires in 2004 for my 2001 Nissan Xterra. After 24000 miles I noted that there were small pieces of the sidewall missing on all four tires - evenly spaced around the sidewall of the tire. On the rear passenger tire, the largest missing piece was approximately . 75" on each side. I did not take pictures and I experienced no driving problems with the tires. I took the car back to the firestone store and they replaced all four tires at no cost. I am sending this because this clearly looks like some type of defect since I was seeing the same pattern on all four tires (and have never seen anything like that before). If other people are seeing something similar, I wanted to have it noted that I also had this issues.