Today 10 years ago I went to Poland to buy a Phone with pre installed #Firefox OS on. The Phone was a Alcatel One, so very shitty. Two years later I installed Firefox OS on my Nexus 5 instead.
It was a very good concept, but sadly rolled out on too shitty hardware so it never caught on.
You’re right the game was pretty much won by then. The Firefox angle was to go for the limited resources end of the market (as it should have run better than Android on less powerful modestly priced handsets) So it launched at the very budget end.
The problems were: People wanted an Android phone so they could use the apps their friends wanted even if they could only afford a slow but cheap experience; the people who might have been interested in a Firefox OS weren’t interested in budget handsets; the experience was poor and there was little ‘app’ support.