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.
I think what destroyed Firefox market share was a RAM leak that took them like a year or two to fix. It consumed all of your available RAM and would bog your computer down. I know that's what drove me away. It took like 10 years for me to come back.