I saw the whole incident... I was waiting to depart on 13R.
The engine fire was pretty serious... it appeared to have gone into complete melt down... molten metal exploding out of the casing at regular intervals and dense smoke.
I must say though that the JFK fire-crews were magnificent!!! The 747 stopped half way down the runway where the fire trucks were already waiting. They started firing foam at the engine within a second of the aircraft stopping... quite impressive fire power too!!!
The evacuation was delayed initially... the first slides appeared about a minute after stopping... all on the right hand side as the port side of the aircraft was completely IMC in smoke and foam by then.
Door 5R slide didn't deploy properly... I suspect that the brisk southerly breeze caused it to be blown under the fuselage before it was fully inflated and it then became jammed in a virtually vertical position. Something to consider for future evacuations, I suppose. The other slides deployed normally.
WELL DONE JFK FIRE-CREWS!!! It was most comforting to see how professional you were!!!