When Firebirds came out, they were FL molds, but they did not call it that, they just called it F, since there was no other mold to differentiate from. Then they made a modified mold, more overstable, with a lip on the edge, and called it FX. Then they began to mark the original mold discs with FL to show the difference. Today, Firebirds are back to just F, but they are FX molds. Also, you can still get FLs, but only in glow CFR plastic.
I love the FL mold, it will actually fly straight, while still being overstable. The FX mold is great for thumbers and tommies. The Champion Edition Firebird (F) is the ONE disc that I actually hoard - I have several of them, NOT FOR SALE, GIFT, OR TRADE!
Oh, also I ain't lettin' go of my last heavy CE Leopard, either... (a lightly used backup to my everyday thrower, and the last remaining CE in my bag