This. I think most Mozilla employees approach stuff this way. I've seen folks leave to form a startup other times too, and that's not because Mozilla is bad at keeping them.

I (non employee; student+volunteer) certainly do this. I've drifted between open source projects and communities, reducing participation drastically when I find something I learn from more. Im pretty involved in a couple projects and I really want to stay because they're awesome, but it's quite possible that in a few years I'll be somewhere completely different. At the moment, I hope not. But only time will tell.