FIRE IN FREEFALL - rare physics experiment

Here's a question, regards to the flame becoming less spherical close to the end, you've not mentioned terminal velocity, if the box had reached it's terminal velocity it would surely stop accelerating and hence no longer be in near zero g inside? With my (admittedly basic) understanding of physics I'd also think that even close to terminal velocity the downward acceleration of the box would decrease, changing the g conditions inside the box?