I'd say WebOS if I have to choose one. I don't know the reputation of WebOS, but given Mozilla is behind FFOS, a lot of the contributors are drawn from existing active Mozilla contributor pool. I don't expect an immediate drop of contributions but I can bet within a year many contributors (paid or not) will put time into other new Mozilla-branded open source projects. Just look at Persona, huge community love, but very little activity (there is always something you can improve but why so few?) Because so few people actually use Persona at large scale (maybe a few), and those who do have to seek for a migration path.