Firefly is green quanta for fireflies, but aflatoxin is more death quanta for you.
One way or another, there are 1 CON and 1 PRO for each one.
Pro for FFQ - It hits more, and isn't so vulnerable to CC. Also, it will keep producing. Upped, the fireflies will give you fire quanta, which is used on that RoF o' yours.
Con - Since it uses greenie quanta, you might have to end up deciding "Do I eat, or do I mutate?" which is kind of bad. Also, if the FFQ dies, you might have a veeery longer game, which isn't very appreciated.
Aflatoxin
Pro - MC (Malignant Cells) They shall never stop spamming. Endless fooder much? It's also great for locking the own FG ^^
Con - HIIIGH Death Quanta, which shall happen way more often then the "Choose mutate, Choose spam", but it changes to "Bonewall, boneyard, or afla?"
Also, afla dispenses the need of boneyard, as boneyard was needed for spamming. You might as well take out the FFQ and Boneyard if you are going to add the aflatoxin and add 2 Afla's and pray you never get low on death quanta.
Or, you could just wait, be lucky, get a grey nymph or two. Now that would be neat, wouldn't it?
Useless Edit : Fire Icon!