Tire sidewalls
 

I was cleaning up my bike after a good ride and I noticed a place in my sidewall. I have Michelin XCR X'trem tires. I got these tires because where I ride there are a lot of roots and rocks and they go pretty good over them. I think a root or rock jabbed my sidewall, no flat though! It didn't go through the tire. I'll try to post a pic. My question is if these tires have weak sidewalls or is this just something that would have happened to any tire. If it's the tires, what are some good tires with strong sidewalls?

Geax Sturdy or Lobo Loco...good strong tires. The Michelins have a rep for weak sidewalls. Can't say that any other tire would not have been cut but who knows. The Geax are better IMHO than any Michelin

I was also told that folding tires will always have weaker sidewalls compared to wire tires. Is that true? Should I forget about folding tires for the MTB?

You shouldn't need to think about that. You could try a tubeless-ready tire, it will probably have a thicker sidewall than a non-tubeless tire.