From a technical point of view, this is just a brilliant and exciting idea! Those technologies are open and standardized. And to start developping your own applications, you just need your preferred text editor and a recent version of Firefox!
I'm very happy with Android 4.2 on my Nexus 4 but I'd really like to start using (and maybe hacking :-)) Firefox OS on a real device (instead of the simulator), unfortunatelly, the compatible devices and the port efforts are quite rare and unstable at the moment. While writing this post, I found that Firefox OS can also be installed on Nexus 7 but the lack of WIFI explained in the last sentence of this page is not very motivating...