The Idle API is used to notify an app when the user is idle. This lets an app take action when the user is doing nothing with his device. The most common use case is to save battery power; in this case, it is commonly used in conjuction with the Power Management API.

API Firefox OS Idle Non-standard Overview Reference

API DOM Firefox OS Idle Method Reference Référence

This method is used to remove an idle observer previously set with navigator.addIdleObserver.

API Firefox OS Idle Method Non-standard Reference