In what might be odd synchronicity, after recent events on DC's Legends of Tomorrow left us with no live-action Firestorm, today is the 40th anniversary of the character's first appearance.
Created by writer Gerry Conway and artist Al Milgrom in the pages of Firestorm: The Nuclear Man #1 released December 6th, 1977 (with a March 1978 cover date), the character was a merging of Professor Martin Stein and student Ronnie Raymond. The initial series was short lived, as the company did a line-wide trimming and the series ended with issue #5. But Conway added the character to the Justice League, he got eight-page back-up stories in the pages of the Flash and in 1982 he got a new series called The Fury of Firestorm. The series would change its name to Firestorm: The Nuclear Man and run until 1990.
During the initial run, Conway left the book and it was taken over by John Ostrander and Joe Brozowski. They changed things up, having Ronnie form with a Russian named Mikhail Arkadin, but it was controlled by Stein's mind. This lasted a little while until Ostrander decided to then make him a fire elemental. This would happen when Raymond would merge with Arkadin and a Russian Firestorm clone named Svarozhich… it was a bit complicated.
In 2004, DC tried a new version of Firestorm with Jason Rusch in place of Ronnie Raymond who died in Identity Crisis. Rusch was created by Dan Jolley and CrissCross. The new series would last 30 issues before being cancelled and Raymond resurrected in Blackest Night. Another series launched in 2011 staring both Raymond and Rusch as part of the New 52, but it only lasted 20 issues.
The character has appeared in multiple animated series, but its highest-profile appearance started on the CW series The Flash where Robbie Amell played Ronnie Raymond and Victor Garber played Professor Stein. That combination last through the end of the first season when Firestorm helped Flash close a breach between two Earth's, but they were split apart while doing it and Raymond is presumed dead. The Ronnie Raymond version of Firestorm made an appearance later in season 2 of The Flash as an evil version called Deathstorm… he, too, would die.
During the second season, it became obvious that Stein had to find someone to merge with or risk blowing up. A replacement for Ronnie was found in mechanic Jefferson Jackson (Franz Drameh). The pairing would go on to become regulars on DC's Legends of Tomorrow until just two weeks ago, when Stein died during Crisis on Earth-X and Jax left the Waverider at the end of the mid-season finale.
It's interesting timing that Firestorm leaves Legends of Tomorrow the night before his 40th anniversary. He is one of the most unique characters created in comics, and I'm sure we haven't seen the last of him in live action.