@Turin:
My greater concern is not that these batteries catch fire, but that they fail! On a fly-by-wire aircraft, that gives a whole new meaning to the term "dead stick". Yes, I know almost everything else has to fail before this matters. But consider Sully's celebrated ditching:
He lost both engines. His APU was not running. If his batteries had failed, and his aircraft had been a 787, he would have had no flight controls. At all.
The 787 also has individual back up batteries for the three ACEs (Actuator Control Electronics-part of the fly by wire system) they are also Li-Ion. The 777 is similar.
According to Ben Sandilands, they nearly fried the Flight Control backup battery as well:
Dreamliner crisis: JAL 787 was at risk of 2nd battery fire | Plane Talking
The Firebox would seem like a good idea, essential in my opinion, if you want to fly Li-Ion batteries.
But as I said before, my problem with Li-Ion batteries is not that they catch fire, but that they fail. I don't think Lithium Ion batteries are sufficiently reliable for safety-critical aviation service.
Cheers