Having spent a couple of years building apps for KaiOS (which is really Firefox OS), I can say that it's probably the easiest development platform out there - mostly pure web; with some proprietary extensions. I wish Firefox OS hadn't thrown in the towel so early. These devices are selling millions of units every month.
firefox os had so much potential, now with pwa's being the next big thing, it seem like firefox os could be a game changer. Too bad they killed it.
> “The full Safari engine is inside of iPhone. And so, you can write amazing Web 2.0 and Ajax apps that look exactly and behave exactly like apps on the iPhone. And these apps can integrate perfectly with iPhone services. And guess what? There’s no SDK that you need! You’ve got everything you need if you know how to write apps using the most modern web standards to write amazing apps for the iPhone today. So developers, we think we’ve got a very sweet story for you. You can begin building your iPhone apps today.” - Steve Jobs/iPhone announcement Jan 2007 at macworld
https://medium.com/datadriveninvestor/2019-the-year-of-progr...
What's old, is new again, 12 years later. It's how it always should have been. And guess what? There's no SDK that you need!! ;)