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.
KaiOS, a FirefoxOS fork, is used in the JioPhone in India. It is a feature phone with some internet capability, and is reasonably popular among lower middle-class users.