io9 reports that Alan Tudyk, who played the pilot Wash in the show, will be playing multiple roles. He has done some pretty diverse voice work for Disney movies like Wreck-It Ralph and Frozen, so that makes sense. At least one minor character will be showing up as well: Michael Fairman as the torture-happy crime boss Niska.
Firefly Online is all about getting your own ship and crew and taking on missions, rather than playing as the Serenity crew themselves. Presumably, though, this news means you'll cross paths with them at some point.
We also got a first look at gameplay in a new trailer, full of Firefly fans and a Jayne hat.