Firefox OS was amazing and, sadly, would STILL be amazing today.

Boot-to-Gecko is brilliant because honestly most apps today... are Web pages packaged in an "app". Most PWA with desktop shortcuts (and ideally offline responsive mode) show that. Very few "apps" genuinely need to be apps.

Consequently being "just" a phone with basic connectivity and delegating the rest to the browser made perfect sense.

I didn't work because it didn't make sense or wasn't technically feasible. It didn't work because anybody who made a mobile OS wanted THEIR own walled gardens. The fact that today we are stuck with Android an iOS shows how needed it was and still is.