If it won't stay down then it should be replaced.
To test the intertia switch Check for voltage on both sides of that safety switch. Use a multimeter ora test light.
If you don't have voltage & you have pressed the button,then the switch is either shot or the connector is bad, etc.
EDIT: found this:
"...With age and especially in climates with extreme temperature changes, the inertia switch can fail all by itself. The spring lock mechanism that holds the button down can rust or become stretched, and the button will pop up at will. I have found that this usually happens on a very cold winter night. Understanding this problem before it happens can be a blessing! If you're stuck somewhere and this is indeed the case, you can temporarily tape the button down securely to get you home. A new switch usually lists for around $80 US and can be easily changed yourself. This may still be a Ford dealer-only item; nevertheless, feel free to shop around. .."
btw, some off-roaders replace the switch with a simple toggle switch; but make sure it is ON when daily driving.