Der Feuerplan was an Imperial German plan for launching a firebombing campaign against Paris, London, and New York City using zeppelins and strategic bombers. Due to the ineffectiveness of conventional strategic bombing during World War I, Imperial Germany developed the 1 kilogram (2.2 pound) B-1E Elektron bomblet in April 1918. While studies indicated that firebombs would be a potent strategic weapon, the Elektron bomb had the additional qualities of being lightweight (due to using a mixture of magnesium and aluminum) and producing magnesium fires that would be made even more intense if water was used to try to extinguish them. Here's more information on the planned use of firebombing against Paris and London, translated from the German Wikipedia article and rewritten for readability: Here's some information on the planned air raid against New York, also from German Wikipedia and rewritten for readability:
I would assume the British and French will react in kind of the Germans are going to fire bomb London ore Paris.