He was the CTO for less than 1.5 years, I guess he didn't like having that position at Mozilla if he left so quickly?
If you're not learning anymore, it's not fun anymore, or you're not having the effect you want to (or any combination of those), its time to leave.
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.