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.
Indeed, there were advantages when It came to app development. There was for example, the Unity web exporter. Embedding web apps for the OS worked out of the box. On the flip side, there was an impact on performance. Like there was no multithreading possible. At least not for the Unity Web export.