If you had followed this thread you would have read the factors that make the fire in flight scenario a less than likely option. Fire cannot deselect transponders, nor call up menu options to shut down ACARS systematically, let alone preselect route 2 waypoints on the FMS.
But that isn't the biggest problem with the fire theory. The biggest problem is that the fire has to meet two conditions.
(a) the fire had to burn such it disabled the power to certain electrical components and not other electrical components despite the fact that these electrical components are on the same power circuit.
(b) that this fire was hot and heavy enough to result in the incapacitation of the pilots yet light enough to burn itself out before effecting any of the control surfaces, cabling, etc so the plane could fly for five more hours.