Well, you can't hide that. What minify does is shortens the names of all variables and objects and removes unnecessary blank spaces and all, to shorten the size of the code. It does also obfuscates the code. But that doesn't affect Strings.
But still that's not enough. That's where the Security Rules comes into play. And you can also restrict your Firebase project to allow Reads/Writes from a specific domain only domains from the Google Developers Console.