It's open source in the sense that the source is available. It's not open-source in the sense of being a community project. As for Javascript vs Java, Javascript has come a long way. Recent JS engines are pretty fast. Not to say that it can't be tricky to maintain a large base of JS code, but that's a different debate.
I for one am looking forward to it, though the lack of apps compared to the other two dominant ecosystems will be a problem.
"not open-source in the sense of being a community project."
Android is no less a community project than FF OS. And it does have a great community of outside developers around it, XDA. Check out the forums.
If you want to contribute back, Google has been good about accepting patches into its AOSP project. For patches they won't accept, there's always CyanogenMod.
"JS engines are pretty fast" Good enough for simple text based apps and casual games, yes. Modern 3D games, not by a long shot. Native code, NDK, or Java under HotSpot are the only decent options creating for modern responsive 3D games.
I for one am looking forward to it, though the lack of apps compared to the other two dominant ecosystems will be a problem.