Note: There is a lot of information here, but if I have provided a link, it is probably something you should click on and read. OAuth is a complex enough subject on it's own, and hacking these two services together only adds to it.
Even so, I have found Firebase's API to be simpler than almost any other OAuth solution I have explored. When all is said and done, the important bits of code related to authentication is actually less than 10 lines. If you have ever tried to implement your own OAuth flow before, you know how amazing that is.
In the end, it may be worth using Firebase for authentication, even if that's the ONLY thing you use it for.