The people defending the fireflies are projecting and going full headcanon to hate Joel. Please tell me who to love now, daddy.
The fireflies were incompetent, ill-equipped and in shambles, they were close to getting wiped out, and hoped a vaccine would stop the military from stomping them into the ground.
It was a political move that was whitewashed into an epic struggle to save humanity that Joel cut abruptly because he's a monster in the sequel.
Depending on who you ask about the recordings' awkward wording, Ellie was the first immune they had found.
It was arguably the biggest chance humanity had at getting a vaccine.
Common sense would tell you to treat her like a princess and take time to test her. What do they do?
They want to cut her open, still unconscious, like five minutes after finding her.
They fail, then what? You didn't even get to see if her offspring is immune.
Over half of humanity was wiped out at the start of the outbreak, there was literally no need to rush.
They were betting on the vaccine bailing their asses in the conflict vs the military, that's it.
In the long run Joel might have saved whatever is left of humanity. He didn't mean it, he did it for himself but TLOU1 works better if you consider both sides could have been wrong/right.