Complex sentence: In the 1970s, he left his post as bookkeeper to Firo, and until the 2000s, he and Czeslaw traveled the world searching for immortals to bring them together again because Szilard no longer poses a threat.
Split into simpler sentences: In the 1970s, he left his post as bookkeeper to Firo. After, he and Czeslaw traveled the world searching for immortals to bring them together again because Szilard no longer poses a threat.