I'm trying to upload data to the storage Bucket of my app but the line
var uploadRef = firebase.storage().ref();
Firebase Storage: No default bucket found. Did you set the 'storageBucket' property when initializing the app?
Turns out the polymerfire-version provided by the Google web-starter-kit was not the latest one. By the time I found that out I already used the firebase initialization snippet provided on the firebase homepage and therefore don't need the element anymore. Now everything works fine.