takes place in the far future way out in space after humans left Earth behind, but it’s never made completely clear what happened to “The Earth-that-was”. Like many other parts of the world of Firefly, before it was canceled, the show’s short run meant that there wasn’t a lot of time to dig into the backstory seriously.
What little information there is about the Earth-that-was also comes from sources that are either heavily biased, untrustworthy or questionably canon (like role-playing game handbooks). This makes it an incredibly tough question to answer, especially with the 500 years that passed between the fleet leaving Earth and the events shown in the series.
Earth-that-was in Firefly seems to have been left uninhabitable, though the reason isn’t exactly clear. At some time around the end of the 21st century, a fleet of ships left Earth to find new planets that they could survive on. This comes from the beginning of the movie, Serenity, which explains exactly what happened to Earth-that-was. The population of Earth had grown so large that the planet could no longer support them, so they had to find a new solar system and terraform the planets in it. While this seems like a good explanation, it’s also presented as a lesson given at the Academy that tortured River, throwing this story into doubt as being more propaganda than fact.
There are two times where direct references are made to the fate of Earth-that-was in the series. First, in “Our Mrs. Reynolds”, Saffron relays a legend her people have about Earth-that-was. Given that Saffron scams Mal into marrying her and the legend is told very erotically, it’s not likely to be a very genuine explanation of what happened to the planet.
The other explanation in the series comes in “Heart of Gold”, where there’s a puppet show in Mandarin in the background showing the fate of the Earth. According to the book Firefly: The Official Companion, the first line of this show translates as “Little by little, the tribes used the Earth up. Barren, she had little left to offer them.” This matches with the Alliance’s story, pointing to the Earth being abandoned because it became uninhabitable.
Unless the Alliance’s scrubbing of history was perfect, this points to there being a massive disaster on Earth leading to a fleet of ships leaving to find a new solar system. That leaves the exact status of Earth-that-was up in the air, as there appears to be no contact between any planets in The Verse and Earth-that-was. If there was contact, it would just be called “Earth” and not be spoken of entirely in the past tense.
The few pictures shown of Earth-that-was also paint a picture of its destruction. While it’s hard to make out too much detail, the atmosphere and oceans are noticeably darker from pollution. While it may not have killed absolutely all humans on Earth, this level of ecological destruction would make survival much more difficult and knocked humanity on Earth way back on the civilization scale. All of this points to Earth being effectively destroyed in Firefly, where even if there are survivors on Earth-that-was, they’d no longer be anything like they were before the disaster that led so many people to flee Earth.
