The thing I like about this platform is that it runs most web technologies. You can thank Firefox OS for that. We used React ecosystem to develop apps for KaiOS and they worked well enough. Jio Phone ships with Video and Audio streaming apps which are all built on either React or Angular and use HTML5. I think all of that was possible on Firefox OS as well and they haven't really changed anything substantially. We were given links to archived FirefoxOS documentation pages when we asked for documentation.
They have managed to package FirefoxOS for enterprise customers. From what I know the KaiOS team comprises of people who worked on parts of Firefox OS (but not the core FirefoxOS team). This makes me wonder why FirefoxOS shut down and why did it not try to market itself like KaiOS has managed to do.