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.
Now it is Kai OS
And it sucks unless you want a feature phone.
commercially, but there’s also https://capyloon.org/
offering a GSI too, so if you have a modern Android lying around you can install it