The DC-10 is real. It was used back in 2007.
http://telstarlogistics.typepad.com/telstarlogistics/2007/08/former-dc-10-ai.html
A 747 water bomber has also was tried in 2009.
http://telstarlogistics.typepad.com/telstarlogistics/2009/08/747-supertanker-makes-its-firefighting-debut-.html
If there was a fleet of 747s on standby they could stop the largest of fires in a hurry, but for some reason they don’t really seem to be interested in doing this.
747 probably unsafe to fly that low and wake turbulence may fan the flames.