Take note.

[fountainhall]

The hold is at the same pressure as the surrounding air, I don't think it can sustain a fire

I'm not sure the comment about fire in the hold is accurate. In 1987 a South African Airways B747 combi en route from Taipei to Johannesburg crashed into the sea off Mauritius with the loss of all lives. The cause of the crash was a fire in the cargo hold which was so hot it disintegrated part of the hull. It is thought that lithium batteries might have spontaneously combusted, but the true cause of the fire was never discovered.

In 1996 a ValuJet DC9 crashed in the Everglades due to fire In the cargo hold. And the whole lithium battery issue was highlighted in 2010 with a cargo hold fire which caused the crash of a UPS 747

[arsenal]

Live animals travel in the hold and survive so a fire probably can too. Ask bkkguy he knows everything apparently.