Firs Island is one of the Endpoints in One Piece. There is a Marine Base that is situated here.
One Piece Film: Z
Firs Island has a large volcano, under which there is a heavily armed Marine Base. This Marine Base has a large bridge that links all the insides of the base. There is strict security around the chamber which has Dyna Stones. Firs Island is a seal for the subterranean magma under the New World. If Firs Island receives any kind of severe damage, it will cause the volcano to erupt and cause large-scale destruction.
After Shuzo was defeated by Monkey D. Luffy and put into Impel Down, the Neo Marines came to Firs Island. After they came to Firs Island, the Neo Marines attacked the Marine Base. This gave Z an opportunity to steal the Dyna Stones which gave him a chance to put his plan of the Grand Reboot into action. While he was loading the Dyna Stones into his ship, Admiral Borsalino interfered and they started to fight. Z used the Dyna Stones to scare Borsalino and also destroyed the island at the same time which caused a huge volcanic eruption.