Firestorm is one of the hardest characters to get right on screen as his origins and powers can confuse some people especially the fact he is two people merged into one.
The Flash tv series has at least cast one half of Firestorm in Robbie Amell, real life cousin to this universes own Oliver Queen Stephen Amell.
Having him as STAR Labs employee Caitlin Snows fiancee is a nice nod to DC continuity where Snow’s villainous alter ego Killer Frost is a major enemy of Firestorm.
Taken from superherohype.com:
Not only is The CW expanding their DC comic based shows with “The Flash” and “iZombie” this year but they’re doubling down on the Amell family as TV Line reports Robbie Amell (cousin to Oliver Queen himself, Stephen Amell) has been cast as Ronnie Raymond for “The Flash.” Raymond will serve as a S.T.A.R. Labs maintenance worker who is engaged to Danielle Panabaker’s Caitlin Snow and reportedly plays an integral part in the accident that gives Barry Allen his powers.
In the source material, Ronnie Raymond is one half of the superhero Firestorm, who he becomes upon fusing together with his partner. The character has some of the most complex and unique powers in the DCU as he’s capable of transforming matter into other elements in addition to superhuman strength and flight. It’s unclear if he’ll take on the role of Firestorm in the series, though the irony of him being engaged to Caitlin Snow (the future Killer Frost) is a nice touch.