Why does Firestar die?

Firestar dies after DEFEATING Tigerstar (Tigerstar does NOT kill him) when he comes back from "The Place of No Stars" after that lightning strikes a tree and sets the tree on fire and the tree falls on Firestar killing him and his last life and later gets picked up by StarClan by the cats who gave him his lives (Yellowfang, Spottedleaf, Brindleface, Runningwind, Bluestar, Lionheart, Redtail, Swiftpaw, and Silverstream) along with the other cats who died which is Hollyleaf, Mousefur, and Ferncloud.

*SPOILER ALERTS BEWARE*