Boot2Gecko or whatever the browser as Operating system was called. This was a project that should have focused on providing whatever its current users needed expanding and evolving to do whatever those users wanted it to do better.
Instead it went chasing markets, abandoning existing users as it did so, in favour of potential larger pools of users elsewhere. In the end it failed to find a niche going forward while leaving a trail of abandoned niches behind it.
For a few short months circa 2016 or 2017, KaiOS was the number one mobile OS in India. This was probably because of all the ultra-cheap KaiOS-powered Reliance Jio phones flooding the Indian market at the time.
I noticed the trend when I was working on a major web property for the Aditya Birla conglomerate. My whole team was pleasantly surprised, and we made sure to test everything in Firefox for that project. But everyone switched to Android + Chrome over the next few years, which was a shame.
Today, India is 90% Chrome :(
I adored my Firefox Phones. Writing apps was so easy I built myself dozens of little one-offs. Imagine if it had survived to today, its trivial html/css/js apps could be vibe coded on-device and be the ultimate personalized phone.
Luckily it wasn't long after Mozilla abandoned it that PWAs were introduced and I could port the apps I cared about.