I would have made that same mistake if I was making decisions for Mozilla at the time. It's such an elegant, logical and tempting ideal to turn the browser into the OS. Too bad it didn't work out.
Well, now we have PWAs. I'd be more than a bit bitter if I was a mozilla dev tasked with making those work on android/ios instead of on firefoxOs.
Good luck getting Apple to allow basic features like storage without weird limits, notifications, and the rest of the list. They killed Flash in favor of HTML5+JS and then decided for the walled garden.