I am all for an informed and empowered user. Notification of poor idle behavior would be great, along with an option to babysit the app (suspend all its threads when not in the foreground).
That said, the behavior of some apps like Firefox are so bad that they deserve to be idle managed by default until the upstream project fixes it. There should be exceptions for when the battery is not discharging or sound is playing. Of course, if you want a pocket warmer, this could be disabled.