Firestar will join StarClan when he loses his last life. yes firestar join's starclan after the battle in fading echoes he had one life then he died of Greencough so brambleclaw becomes bramblestar also Jayfeather visits StarClan and the Place of No Stars, when he goes to the Moonpool and sees the faint outline of Firestar. Yellowfang explains he will be able to see and hear when he fully dies, and they see the faint outlines of other cats, "dead and long forgotten". that's what she said to jayfether so finally he could become a warrior but he needs a apprentice.
It is not confirmed when, but it will happen in Omen of the Stars
Firestar escorted Bluestar to StarClan when she passed away. He was a close friend and former deputy of Bluestar, and he guided her to join the ranks of the ancestors in StarClan.
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.
At the beginning of the Series, Firestar will be leader. If Firestar does join Starclan in this series, it does seem likely that Brambleclaw will succeed him as leader. Some other possibilities include:LionblazeGraystripe
I think Firestar dies in the big battle agianst StarClan and the Dark Forest..
Graystripe is not dead yet, when he dies he will join StarClan.
Tigerstar kills him in the dark forest vs starclan battle in the omen of the stars last book
Firestar made Brambleclaw deputy because Leafpool had a sign from StarClan saying he was the right choice.
Firestar was the main point of StarClan's prophecy, Fire alone can save our clan. He helped drive out BloodClan from the forest.
So far in in the entire series, Firestar has not joined Starclan, In Omen of Stars (the 4th book section) he might die, but no one really knows who many live he has left. But all that may be answered somewhere later in the series.
After she died, Bluestar joined starclan.
Whitestorm was a senior warrior, meaning he had lots of experience. He was a supporter of Firestar when Firestar was deputy, and he had an unshakable faith in Starclan. When he was appointed deputy, Firestar commented that Whitestorm's surprise at getting the position was part of why he was chosen.