The main problem with Firefox OS was that it was really slow. At the same time it was targeting budget phones.

But on the other hand progress was quite good. Back in the days I was maintaining unofficial images for Alcatel Fire. Each version was a little bit faster, but you really can't do much when the whole OS is a browser running on a device with with 256MB of RAM and a single core CPU.

I use it as my primary phone for 2 years, first with the Flame, then with a Z3C. For me Firefox OS was the finale move of Mozilla, either it successes and Mozilla becomes a major actor again or it fails and they slowly die. And thebmy decided to kill it right when it was becoming stable enough.

As with most new operating systems, its biggest problem was lack of apps. Mozilla seemed to abandon Firefox OS right as Progressive Web Apps were starting to take off, which would have done a lot to fix that problem.