Firefly's regulars included four actors who'd been guests on Buffy or Angel, three of whom had played recurring roles...
Actually, I think it's the other way around, at least with three of them. Summer Glau of course had a guest episode prior, but I think Nathan Fillion and Gina Torres' recurring roles came after Firefly
- late in Seasons 7 and 4 of Buffy
respectively. Adam Baldwin's role on Angel
Season 5 definitely came afterwards.