The only thing that has a chance of happening is some external party coming up with a different protocol or extending the existing one, and gaining traction as

The only thing that has a chance of happening is some external party coming up with a different protocol or extending the existing one, and gaining traction as an identity provider. Then, like we did with Pocket and "Save for Later", it's not unreasonable to think that Firefox would accept a system add-on that is compatible with this protocol to ship an in-browser UI. "gaining traction as an identity provider" being the hard part, of course.