You can't discount the fire theory completely but it's would be a hell of a coincidence for it to play out in a way that would match what data we have..it's early but if you want to understand why many who have relevant knowledge don't believe the fire theory you'll have to bear with me:
From what we can gather everything on the flight seemed normal until the first point enroute where there was slight weakness/seam/gap in ATC coverage - the handover at the border between Malaysian and Vietnamese ATC.
It's not uncommon in many parts of the world, especially overwater, to have a short period when you are not in full contact with ATC and not radar identified ... anybody with route experience in that part of the world would know that if you wanted to go "dark" that would be an ideal point to turn transponders etc off. They'd also know there would potentially be confusion between ATC agencies if you went missing at that point and if you look at the report you'll see that certainly happened...
Heck of a coincidence if the fire happened just at that point and caused the confusion.
A few minutes later for a short period an aircraft is picked up on some Primary radars crossing the Malaysian peninsula on a south easterly heading pretty much along the line of the Thai Malay Border and then turning north west to track up the centreline of one of the airways that runs up the strait of Mallaca.
It would be a heck of a coincidence that the aircraft could do that all by itself having had a fire in the avionics bay and with the crew incapacitated.
Having flown outside radar coverage again aircraft is still communicating at a basic level via satcom for several hours and appears to have tuned again once well off shore and headed south...
Heck of a coincidence etc....
I'd agree you can't completely discount a fire but in terms of probability how do you honestly think it rates verses the theory that somebody onboard the aircraft with technical knowledge (aircraft and ATC) onboard the aircraft was behind all this?