In my opinion, it's a race between FirefoxOS phones and Ubuntu phones as to who gets 4G first. I've been using FirefoxOS phones for a while (Open, Open C and now Flame) but I'm missing the 4G connection, and that may well be what tempts me away from FirefoxOS.
It's in the mainline B2G repo, so you should be able to follow the instructions at https://github.com/mozilla-b2g/b2g to get a build. Use `./configure.sh aries` to configure for the Z3C. Other supported devices include the Nexus 4 and Nexus 5.
