I don't use Firefly so I don't know how to make materials that work with it. Inside the "SkirtLength" node there's just a bit of math plus a "V" variable node which I think is Cycles-specific. If anyone knows the Firefly-equivalent for this, it should be possible to modify this so that the variable length also works with Firefly materials.

-- I'm not mad at you, just Westphalian.