IMO FirefoxOS wasn't well received partly because it wasn't Android-based. Ultimately I wouldn't try to do a mobile OS without ~$10B in marketing/ecosystem funding.
Agreed, it needs to be Android based - the other option would be to implement iOS APIs, but Android would be a lot easier.
An analogy would be if you make a browser, to use standard HTML and HTTP, not try to invent your own thing.
Its fork, KaiOS, is widely successful (but Proprietary). They should have stuck with it.