There is no 100% secure solution to provide convenience to end users.
I chose what I think is the best tool to store posting keys.
It would require Firebase servers to get hacked for keys to leak.
They build their reputation on the security of their customers data. And they are accountable.
I trust them probably more than I trust Steem Inc to secure much more sensitive information than the posting key.
RE: SteemLogin - a new and easy way to sign in to Steem!