KaiOS is not Firefox OS. Our platform is based on the original Mozilla project. We even have people from the original Mozilla team in our engineering and UX departments. But KaiOS has been developed into something much more robust and expanded than the original Firefox OS. Think of us as distant cousins, not siblings nor children.
Source: A Short History of KaiOS - KaiOS
I had no idea that Firefox OS lives on (and in feature phones of all things). I hope it goes well for them!