Pictures from the scene show damage to the top of the fuselage just forward of the tail. The cargo doors were closed but passenger doors on both sides (doors 3 & 4) are opened. This area may be the aft galley of the aircraft and not near the APU lithium ion battery.
This fire does not appear to be related to the lithium ion batteries given the location. This particular aircraft is ZA261 (LN44, ET-AOP) and was delivered to Ethiopian on Nov. 20, 2012.
The flight arrived around 6:30 AM London time and was parked at a remote stand as it's return flight to Ethiopia was not due to leave until 9:00PM. The fire occurred at about 5:15PM, almost 4 hours before departure. At that time the aircraft should be empty and completely powered down. Again it is too early to say what happened but I am leaning towards this NOT being a battery fire but perhaps some device in the galley being left on. The investigation will determine the final cause.
As to repair and return to service of this particular aircraft, that will be determined by the airline with Boeing's help. It all depends on the extent of the damage to the fuselage.