Firefox OS had serious issues.
* Web standards 2013-2017 weren't ready enough.
* 2013-2017 phones still weren't powerful enough for complex JS apps to feel fast.
* asm.js was de-facto proprietary (a new FFOS with wasm would be be another story)
* The UI wasn't so great.
* Their launch devices were slow, cheap, and sucked.
* Their launch devices weren't readily available to developers.
* Their OS provided no real advantages over iOS or Android
The OS is still around as KaiOS (with a couple hundred million devices shipped IIRC) and I believe it still powers Panasonic TVs.
Interestingly, I think a FirefoxOS of today with good React Native and Flutter integration and cutting-edge WASM support could have a shot at success if not completely mis-managed.