In the Mountains and Farmlands regions I have shot down 6 Firebirds and only been able to find one of them to loot. The others for some reason are NOWHERE to be found. This includes shooting them down above an open field. I play on Steam PC. I only have RARE occurances of crashes so my computer is not the issue.
I too have run into this on occasion. One time, I shot one down over a field, but I stumbled onto it in the trees, on a hill…
This has also happened to me twice even when they fall on land. Also make sure you stay away from the water when fighting the Firebirds. If they land in the water after having been taken down you won’t get the rewards, unless it landed close to shore.