On WoBP, Firefly x Firefly produces:
Percent Fraction Morph 6.25% 1/16 Normal 12.5% 2/16 Fire 12.5% 2/16 Pastel 18.75% 3/16 Firefly 6.25% 1/16 Super Fire 12.5% 2/16 Pastel Super Fire 6.25% 1/16 Firefly 6.25% 1/16 Super Pastel 6.25% 1/16 Superfly 6.25% 1/16 Superfly 6.25% 1/16 Super Pastel Super Fire
For a snake with multiple morphs, like a Firefly for example, you would click 'Fire', then hold down 'Ctrl' on your keyboard and click 'Pastel' so that they're both selected. If you just click on the second morph, it deselects the first one.
Personally, I'm greedy... I want a Super Fire with Pastel and Yellowbelly. But I don't think you can actually see the Pastel or Yellowbelly on a Super Fire. It would just be in case I decided to breed him. I think that would have to be a Super Fire from Ultrafly x Ivory Fire, to be sure. Hmm....