The only way to get the M&P to fire is to pull the trigger. You can throw it fully loaded and the internal safeties will prevent accidental discharge.

Using a holster that covers the trigger and keeping the finger out of the trigger guard until the target is acquired are vital to safety.
Also, avoid any holster with a retention strap, as the strap can get inside the trigger guard when reholstering. The kydex Comp Tac and Blade Tech holsters are optimum for the M&P, being adjustable for retention force and allowing quick draw.
Unlike some designs like the 1911 where the thumb safety is an integral part of the design, the thumb safety on the M&P is simply an add-on for those that require/want an exteral thumb safety.

__________________
Science plus Art