To my untrained eye, that fire is in the cabin...
Looking at post #7/pic 2, I agree with the above. The flames are coming out of the opened emergency exits (on both sides there is one aft just before the engine inlet, and two overwing), there is nothing visible on the photos that would indicate an engine (or even APU) fire.
Note however that the fuel lines run under the cabin floor, with the pumps on any unnoticed fire fed by ruptured fuel lines would quickly spread under the cabin. The old Tu-154s had plywood floorboards (I believe the M has fiberglass boards), not exactly a fire retarding material...
Whatever happened there, clearly there was a massive sustained fire from the start that quickly spread along the length of the fuselage. The fireworks/vodka combination in passenger luggage is not an entirely implausible one, given the time of the year...