The FirefoxOS legacy is alive in KaiOS which has 50M+ users now and is growing. Indeed a very poor decision from Mozilla's leadership to kill the effort too soon.
I have KaiOS on a Nokia phone that I intended to use for email and 4g hotspot. I didn't expect much but it literally a POS, super slow, impossible to customize (i.e. hotspot quick launch), and no apps in the marketplace. It would be more usable with a pre symbian OS in my opinion. People that rave about KaiOS I figure are like that those that endless recommend SICP or 'Clean Code' as the book of gods and probably never read it.