I'm trying to retrieve Firebase data with Polymer Fire. When I look in the console it's returning two objects, however the length of the array is three. When I'm trying to execute a dom-repeat I'm successfully printing two filled in rows but also one empty row. How is this possible?
Firebase stores data as associative arrays, essentially a dictionary of key/value pairs.
That means that in order to deal with arrays, it converts an array to a dictionary when you store it and then back to an actual array when you read it. Here you are getting bitten by the SDK converting your non-array into an array by padding it with a leading element.
If you don't want the SDK to do this conversion, the easiest way is to store the items with a non-numeric key, e.g. "item1", "item2".
Read more about how Firebase deals with arrays in this classic blog post: https://firebase.googleblog.com/2014/04/best-practices-arrays-in-firebase.html