As of now, Firestar has not lost his eighth life.
Firestar lost a life in the battle of bloodclan when scourge clawed his head.
It is believed Silverstream gave Firestar a life because she was close with him and Graystripe. Firestar helped her and Graystripe many times, so maybe she wanted to repay the favor.
He died of greencough.
Lionheart
Silverstream gave Firestar a life because she wanted to show her support for him as the new leader of ThunderClan. She felt a strong connection to him as a former mate of Graystripe, and believed in Firestar's ability to lead the Clan. This act symbolized her trust and faith in him.
Brindleface gave Firestar the gift/life of protection. For kits, the need to defend your Clanmates, the vicious protection a cat gives for their loved ones. It was unbearable to stand for Firestar.
Russetfur, and Firestar loses a life.
Firestar hasn't died yet.
Firestar loses his last life in the last book of Omen of the Stars, or so I've been told. Erin Hunter has said that she doesn't know how she is going to make Firestar's death.
Firestar is on his last life in the forgotton warrior so he will die in The Last Hope. Sad but true. ~DOVEBUMBLE FOREVERZ!~
Russetfur and Firestar lost a life -Jonathan