Lithium-ion batteries are well known for their chemical
instability which can start 2000F fires. Several Tesla cars and other
vehicles burn so hot nothing can put out the fire. Foam and water simply
turn to steam.
Here is a page from the cargo manifest - listing 1,990 (Kg?) chargeable
weight for a shipment of lithium-ion batteries.
1,990 is in kilograms equals 4,387 POUNDS of lithium-ion batteries - more
than 2 tons.
Some may counterclaim that smoke detectors would alert the crew to a fire.
But perhaps not. Maybe smoke detector(s) malfunctioned on the flight,
the fire destroyed the fire warning system wiring or smoke and detectors
did could not detect the fire where it broke out.
It's very hard to comprehend the market value of this big lithium battery
shipment. A aircraft fire would give both the lithium-ion battery industry
and the manufacturer a bad reputation if just one related fire brings
down a aircraft. Perhaps this is why the cockpit voice was edited? Without
a doubt, a lithium fire is hot enough to ignite aluminum all aircraft
is constructed of.
In the USA, it is FORBIDDEN by FAA regulations to ship any lithium-ion
batteries on a commercial flight due to the threat of fire.
Aluminum melts at 1,221F. Lithium burns far hotter, around 2,000F. Aluminum
will catch fire if heated hot enough.
From a scientific paper we learn what can happen to any lithium battery:
If the battery is undergoing a thermal runaway reaction then a explosion
or fire is inescapable. In normal use of the battery, the temperature
is below 40C. But abusing a cell with a short circuit, overcharge, apply
reverse polarity or expose to high temperature, will cause the temperature
to rise quickly. When the temperature exceeds 66.5C, more reactions occur
and generate more heat to quicken self heating reaction.
Thereafter, if the temperature is over 75C, then the reaction will continue
with no return to normal. With increased chain reaction gas and heat are
accumulated. Once the inner pressure exceeds the battery endurance, explosion
is inescapable. Fragments can easy be ignited and cause fire. 
NOTE: Temperatures over 75C is the run-away temperature for lithium batteries
- just 167F. Cars, vans and trucks can almost reach this temperature under
a hot summer hot sun.
Even if the flight crew discovered the fire in time, it's unlikely they
could get close enough to put it out. Note the FIRE warning stated on
the manifest for the shipment below:
Source of manifest: http://www.zerohedge.com/news/2014-05-03/voice-recording-missing-flight-mh370-was-edited
 - http://www.iafss.org/publications/fss/8/375/view