"Now that we have our v1.0 behind us and we’re moving forward with even more partners, we’re going to do our best to bring Firefox OS back into our heartbeat and will make quarterly feature releases available to partners along with six-weekly security updates for the previous two feature releases. As far as I know, that’s the most aggressive mobile OS release strategy out there (and may still require some tweaking)."
https://blog.mozilla.org/futurereleases/2013/07/19/mozillas-...
You have nonetheless to consider that is the manufacturer who holds responsability for the avalaible images.
Firefox OS 1.1 was delivered by Mozilla on October 9th 2013.
https://blog.mozilla.org/futurereleases/2013/10/09/firefox-o...