Firefly III can export transaction data to CSV files. Please note that this is not meant as a way to backup your data but rather as a way to export your data into another financial system. Of course, I cannot imagine why you should do so ;-).
Under “Import and export” you will find “Export data” where you can export your data. The export can also include old imports and attachments, if you want to.
Please note that the export can take some time.