There's no official word on which episode will be his last, but he does not intend to return for any more episodes in the third season after he leaves, despite keeping his series regular credit on the show. At least that likely means he's not getting killed off? It doesn't appear to be an acrimonious split, however, so the chances of seeing Garber again are high.
Garber plans to make good use of his new free time with a return to Broadway in a revival of Hello, Dolly!
The question now is how the show plans to allow his character, Martin Stein, to gracefully exit the show when he's one half a whole where Firestorm is concerned? When Stein and Jefferson Jackson (Franz Drameh) combine their bodies and minds, they can tap into the pyrokinetic powers that make them Firestorm.
One would assume that they're going to need a new character to step into Stein's place, much like Jefferson stepped into Ronnie's (Robbie Amell) — Stein's first Firestorm partner — place after he was killed.
Legends of Tomorrow airs Tuesdays at 9/8c on The CW.