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.
FFOS was an html mess. The GUI didn't have much to offer. You couldn't organize your apps since they were only accessible through the cluttered app drawer.
The HTML was not the problem, the never finished OS was one yes.
I still liked it because of how easy it was to develop apps for it like I did with my https://jeena.net/feedmonkey
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.