Saving users data without requiring a login can be a tricky problem, but with firebase and a framework that supports routing it doesn't have to be.
I have a friend who has a site called http://www.infantchart.com/. He mentioned one time that people don't like to have to login, or link to a google account so they can save their data. I think we've done this so many times and we understand the reasoning why, that it was hard to understand why someone wouldn't want to do that.
I have been playing with Firebase a ton, and I love it. Just last night I wrote a VERY simple app that adds elements to my data store. I had nearly forgotten that when I add a JSON object to my store, it'll generate a unique id (like a primary key in a typical database). Having worked with Angular JS, I know you can use routing pretty easily. So I now have a solution for his problem. When a user enters in some data, generate a "new" key for them, and redirect to that page for any further input. Now when the user visits infantchart.com/[unique id from firebase] using the ability to route, you simply open a firebase connection to [firebaseurl]/[uniqueid]
How do we get the user to go to that site though? They're probably not going to remember HvawoinvNOINRV to be able to get to their data. Adding a "share" option and really highlight it so they share with friends, this will attract new visitors, as well as help them find their way back. More than likely if they can't remember the path, they'll remember they shared it with friends and find the link that way. Additionally you can store that unique id into local storage so when they visit from the same device it automatically goes to the right link, this way they can always find their way back.
Giving users the ability to save their data without having to actually login/link will help retention and reuse, resulting in a more useful site and a little more intuitive I'd like to think.
Disclaimer: Don't use this approach for sensitive data since finding other peoples data is a matter of cycling through different combinations (it might be a pain and hard to find stuff, but it's not impossible).
Do you have other ways to save user data without requiring a login, love to hear about em in the comments.