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.
I always thought it'd be more of a feature phone type os. Couldn't compete with what Android had to offer to the mainstream Western market at the time using primarily HTML, but I'm glad to find out that is what it turned into.
KaiOS runs on feature phones, with some advanced stuff like WiFi, 4G net and an app store. It should run on low-end smartphones, but I don't think any have been released yet.