To sum this up... if there is a new trigger and if it is anything like the FSS kit from APEX, then your M&P will need to have the thumb safety. That makes perfect sense to me, since I have installed the FSS kit on one of my M&P's, the 40fs, I have decided to not CCW it. Reason being is the original trigger design is what allows us to safely carry our M&P's without a thumb safety, the long trigger travel, the heavy trigger release is all we have from a discharge. That is the way I see the original design. If S&W is saying the new trigger only works with the thumb safety, I believe this to be the reason. The FSS trigger has almost no take up and it goes bang...
I have APEX kits in my other 3 M&P's, living in Mass, our guns come with a 10lb pull, we have to modify our triggers. I installed the DCAEK kits in them, it gave me around a 6-7lb pull with no gritty feel. But I will say, in spite of installing the APEX kits, my M&P's still shot just as accurately and were just as much fun stock from the factory. Had I not read about the APEX trigger kits, I would have been just as happy with what I purchased from the factory.