Firs Island has a large volcano on the island, and underneath it houses a heavily armed Marine Base, containing a large bridge linking to the inside. Inside the heavy duty security chamber is where the Marines stored the Dyna Stones.
As one of the three Endpoints, Firs Island acts as a seal to the vast quantities of subterranean magma underneath the New World. Should sufficient damage be inflicted onto this Endpoint, it will cause the volcano to erupt in a very devastating burst.
Upon landing on Firs Island at nighttime, the Neo Marines attacked the Marine base on this island, plowing a way for Z to steal the Dyna Stones, which he requires to execute his Grand Reboot plan. While loading the weapons of mass destruction onto his fleet, Z was intercepted by AdmiralBorsalino. A brief but intense battle taking place, Z used one of the Dyna Stones to scare off Borsalino, and destroy the island at the same time, causing a massive volcanic eruption.
The island's name is a pun: When read with Japanese suffix "-to" (島,island?), it can be read as the Japanese pronunciation of the English word "first" (ファースト,fāsuto?). This refers to it being the first Endpoint to be shown.