Chuck:
Might be an individual gun/tolerances thing, but the thumb safety on my M&P40C (it was an M&P9C until I swapped frames, but you know that) has always been fine.
I agree that it's essentially useless, but a rat tail file and a little work (and the right roll pin punch!) will stiffen it up a bit so it stays put.
The only problem I've noticed is that the levers are located where I pretty much have to shut the thing off to move the slide - racking a first round, press check, etc. Minor issue....
In my case, as a 1911 guy, I sort of like the commonality, but only because I like to
use the safety lever on the range on both the M&P and a couple 1911's.
jabate01:
Chuck's comments notwithstanding, S&W says you can't.... There's a cutout in the plastic frame portion that's not there, but you could make that yourself (on both sides). I'm not sure if you could get the associated parts, or if they'll work with the sear block you have.
Worst case, I suppose, would be to swap the gun with somebody who doesn't want a thumb safety....
Mine did NOT need to be held down - whether as a 9mm or .40 - but I have heard that some of them are very loose, and mine's not all that tight.... I'm more concerned about having it on when it should be off, but being a 1911 guy, flicking the safety off is kind of automatic. I don't find it of any non-training value....
But that's just me, and YMMV
....
Regards,