The world that Naughty Dog built for The Last of Us is a rather complex one, where many factions form and disband over time, and where power and control of it are not solely held by a single group. This was not always the case, however. In the early years after the outbreak of the Cordyceps Brain Infection, the remnants of the government of the United States, the Federal Disaster Response Agency (also known as FEDRA), became the leading authority in the country. FEDRA imposed martial law after it assimilated the military forces of the nation and formed Quarantine Zones in safer areas, like Seattle, Boston, and Pittsburgh, to both prevent the fungus from spreading further and to affirm its authority.
However, this is what caused Marlene to create the Fireflies, as she felt that an opposing group should stop FEDRA's tyranny and hard laws. The Fireflies fought against FEDRA to restore the pre-outbreak government that the United States had, and the conflict did end up with FEDRA losing some of the Quarantine Zones, especially because more factions formed and attacked the group. The Hunters and Washington Liberation Front are prime examples of this. The WLF managed to regain control of Seattle, whereas the Hunters scattered and tried to hold their territories by hunting down other people for their possessions. Then, in The Last of Us 2, Dina also mentions the Ravens as another faction that emerged to fight FEDRA off in New Mexico.
The Fireflies' Purpose in The Last of Us Part 3
This context is important because of the events that unfold both at the end of The Last of Us and The Last of Us 2, and they are linked to the Fireflies on many levels. In The Last of Us, players control Joel as he tries to smuggle Ellie out of Boston and into Firefly territory, which eventually translates into the massacre of the rebel group at the hands of Joel once he finds out that their plan for a cure of the fungus is to basically kill Ellie. After that, with Marlene and most other members dead, the Fireflies are no more and the remaining living individuals split and go their separate ways to survive.
However, after a few years, it is discovered that the Fireflies are regrouping on Catalina Island, in the Santa Barbara area. This is where Abby and Lev are headed just before getting caught and enslaved by yet another faction in the world of The Last of Us, the Rattlers. Still, at the end of the game, Abby takes Lev on a boat after her fight with Ellie, their destination unknown.
This poses the question as to why the Fireflies were regrouping, though, and what their purpose might be after all these years. FEDRA lost control over most of its territories and, while it remains a notable military force and authority, it is nowhere near its glory days all the way back in the early 2010s. Furthermore, with other factions running rampant and claiming safe areas for themselves, it is tough to imagine what the Fireflies could possibly want.
The question remains unanswered, which means that maybe, the Fireflies might not be needed anymore. And yet, Naughty Dog dedicating part of Abby and Lev's story to finding the Fireflies indicates that the faction might be a focal point of the narrative for The Last of Us 3 if it sees the light of day. The Fireflies' purpose and goal remain unknown, but the group could likely go back to its roots and try to eradicate FEDRA once and for all, even though it would mainly be out of old rancor rather than need.
The Last of Us 3 is not currently in development.