Jupiter is mostly hydrogen, so they'd likely weigh as much as hydrogen. Radius of the Moon is 1737km.Larklight said:
V = 4/3πr^3
= 4/3 X π X 1737^3
= 2.19527062e10km^3
= 2.19527062e+19m^3
Hydrogen weighs 0.0899^2kg per m^3.
M = 2.19527062e+19 X 0.0899^2
= 1.77421991e17kg
The heat of combustion of hydrogen is 144000 kilojoules/kg.
E = 1.77421991e17 X 144000
= 2.55487667e22 kilojoules
= 6.10630179254 petatons
Final Results
Thunderhead creates moon-sized fires = 6.106 petatons