Currently seems like only files can be uploaded since 'data_url' is passed to the uploadString function from firebase SDK. This could be handled by using uploadBytes or by setting the file type argument to 'base64' instead of 'data_url'. The major question here would be how to best convert a dataframe to a bytestring... guessing that's not so hard.

Reference: https://firebase.google.com/docs/storage/web/upload-files

My current solution is to just write the data to a temp file before upload and then delete it afterwards, but that's not ideal. I may tackle this and submit a pr if it really becomes an issue.