How do you define success? This is not a competitor to iOS/Android, it's for cheap low-end devices, which is still a sizable market. Try running Android on 256MB of RAM...
I still have a ZTE Open C with me. It has 512MB of RAM and 2-core CPU.
It's been running on the Firefox OS quite slow. Having in mind a very poor functionality, I considered to switch to Android.
When I installed Android, it was a total game changer. It was fast and smooth (back in 2014 of course, not now) and I never installed Firefox OS again.
Success: Enough market share to build a sustainable business so that it doesn't go away at some point.
And I know about Android's problems there (and mentioned it). But Windows Phone and FirefoxOS both chased those markets and they're both basically gone now.
It's been running on the Firefox OS quite slow. Having in mind a very poor functionality, I considered to switch to Android. When I installed Android, it was a total game changer. It was fast and smooth (back in 2014 of course, not now) and I never installed Firefox OS again.