Who are the Fireflies and FEDRA in 'The Last of Us'?

If you're watching The Last of Us, you would have heard the words "Fireflies" and "FEDRA" thrown around in the first episode. But who are they again?

The two groups are featured both in the Naughty Dog game and the HBO adaptation by Chernobyl director Craig Mazin and The Last of Us creator Neil Druckmann. Now, it'll be...