I think we need a bit more information. Right now ... "just get the user to paste in their token somewhere" ... would be an acceptable answer, but I bet its not the one you want.
- give us a paragraph or two explaining the whole process of what you are trying to do
- whats firebase?
- whats the token for?
- is the user logged into Moodle? if not where are they logged in?
- is it one token per site? one token per user?
I am going to take a guess that you want to authenticate users somehow. And that maybe you want to look at auth plugins.
But you might also want to look at webservices.