I figured it out myself.
First, take an Excel spreadsheet and clean it so that the first row contains the headers and the rest is the actual data. Remember that Firebase keys (which correspond to the column headings) must be encoded in UTF-8, which means they cannot contain it. $ # [] / or ASCII control characters 0-31 or 127.
Then save it as a CSV file. For example, in Excel 2013 it would be: File> Export> Change File Type> CSV.
Then visit various resources on the Internet that can convert CSV to JSON. The one I used was: http://codebeautify.org/csv-to-xml-json
Finally, grab this .json file and upload it to the Firebase database. Voila!
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