Note that there is KaiOS -- popular in India and Brazil but under the radar on HN because it's currently mostly aimed at feature phones. It's based on FireFox OS (hence probably closer to PalmOS than Windows Phone): https://en.wikipedia.org/wiki/KaiOS
That was what I posted in the Firefox ( or was it Edge ? ) discussion, Mozilla could have had Firefox OS being the 2nd most popular OS. Instead they handed it to KaiOS.