firebase.com is our legacy site that contains information about the Realtime Database, Authentication, and Hosting solutions from 2012 until 2016.
After our announcement at Google I/O 2016, we've switched to
firebase.google.com, which contains information about the updated versions of those features, as well as our latest and greatest additions (storage, crash reporting, analytics, etc.).
Unless you're looking for information about our SDKs prior to 3.x, I recommend using the
firebase.google.com site.
Thanks,
--Mike