FIRSTBK on a credit report: bank account or loan
FIRSTBK is probably a bank account or loan: it runs a name, "FIRST", into "BK", the usual abbreviation for bank.
How to find out which company reported FIRSTBK
Your free report at AnnualCreditReport.com shows the full name and address of the bank behind FIRSTBK; our guide to identifying a creditor explains how to ask the bureau if it doesn't.
FDIC-insured banks can be looked up by name in FDIC BankFind.