In an untold tale of decades past, the counterparts of Earth-One's Ronnie Raymond and Professor Martin Stein nearly met, although their costumed careers would not cross paths. Surprisingly, this was when those cosmic twins from Earth-Two had both migrated to Ronnie and Martin's world temporarily.
Rich playboy Rodney Reilly decided to establish himself as a costumed crusader like many mystery men of his day, going by the alter ego of Firebrand. Although not possessing any inherent powers himself, he had a propensity towards making a fiery entrance when faced with the criminal element. Eventually, he and his personal trainer and assistant Slugger Dunn joined the Navy. This left his alter ego available for his sister Danette, who after having been mutated into generating actual flames, became the second Firebrand. As a member of the All-Star Squadron, one of the heroes she had encountered was the Ray, who's alter ego was reporter Happy Terrill.
Back to actual history, after witnessing the departure of members of the All-Star Squadron for Earth-X to join Uncle Sam's team, Rod longed to accompany them. And he obtained the means in the later part of the 1940's when Dan Richards aka the Manhunter returned with his android dog from that world, brought back by the dimension-traveling Manhunter androids that previously recruited him. These robots sent Firebrand to Earth-X in order to infiltrate Nazi high command there who were winning the war, when in fact the hero was helping to vacate positions of power in that regime that would be replaced by the androids themselves who would conquer that world for decades to come! Thus, Firebrand would be delayed in meeting the Freedom Fighters he had so longed to join, until meeting years later.
As for Ronnie and Martin, they eventually accepted their symbiotic nature following the nuclear incident that melded them together which formed a potent powerhouse in their alter ego, even gaining enough acclaim to join the Justice League of America for a period of time. Among Ronnie's most persistent adversaries was Cliff Carmichael, counterpart of the Dark.
During the time when Firebrand and the Freedom Fighters were briefly living on Earth-One where Firestorm operated from, the Nuclear Man never had occasion to meet his counterpart, whom perished when an old Nazi foe of Reilly's tracked him down and exercised revenge on his as the Silver Ghost. Firestorm himself would carve a niche as the preeminent protector of New York City battling a host of foes while earning the respect of the Greatest Heroes of his world. A charming mixture of an enthusiast young man and an analytical older gentleman, indeed!