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.
It's what killed Windows Phone. There was a period of about 6 seconds to get in on the commercial phone OS game, and it was long gone by the time Windows made a legitimate effort (Windows Mobile phones didn't really count - they were stuck with legacy PDA software).
Honestly, the AT&T exclusivity and the late rollout of the app store (iPhones initially only had the factory apps) were the opening that let Android in.