In terms of regular users, there are tons of dirt cheap chinese no-name android devices that more or less work. For 100 euros, you can get an acceptable Android phone from a major manufacturer (LG, etc). FirefoxOS was competing in a similar price range, however offered much less to the end user. I guess it's fair to say that it didn't deliver any extra value.
When I was using FirefoxOS and poking around the code base, I saw potential in their web-first platform as an introduction to programming. It's much easier to write some basic HTML, CSS and JS than to figure how to do the equivalent in Java for Android, etc.
However, things like ionic or phonegap and reasonably good, and it's hard to compete with them as they produce something fairly acceptable and available to run on the vast majority of the smartphones.
At the end of the day, I really appreciate Mozilla's work on this project. Thanks to all the volunteers who contributed to the project. You are amazing people :)