Thank you for posting this, never realized such a thing existed. Makes it easy to maintain different identities. It's made by Mozilla too. Wonder why it isn't a core feature of Firefox.
They try to develop most new features as extensions first these days. It keeps the release cycles from impacting each other and lets them make sure that the extension APIs remain useful for other developers. If it turns out to be a good feature they'll either bundle the extension or build it into the browser from the extension. That's not always something people celebrate though, see the pocket stuff.
It probably will be. Mozilla frequently moves extensions from Labs (which existed then got cancelled, and now has been reinstated iirc) to core Firefox.
It has been a core feature since v50, albeit with less feature than the addon. It's still disabled by default, just need to toggle the following configs to enable it:
