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...
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.
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.