As far as we know, Firefly isn't considered a megafandom unless someone brings us evidence otherwise by the end of March (we're locking the Megafandoms List by then in time for nominations until the end of the exchange when it reopens again).

On fanfiction.net, there's been at least 100 stories posted within the last three months.

As for your original question, it is something I would need to discuss with my co-mod.