Indeed you can self-host and use the owa API to let user install your hosted or packaged application. This will not let you install privileged apps though since they have to be reviewed.
so even Firefox OS is a walled garden? Telcos controlled what can be on your phone, Apple was able to wrestle them and overtake that control - control means money. Google follows the suit, forcing their in-app billing etc... Why would Mozilla support that ugly ancient "tradition"?