Database is encrypted in the local storage of the extension. So far Firefox is secure enough so this is not accessible from outside (it is encrypted anyway). The encryption is using the crypto web API (native from web browser) to use PBKDF2 key. It is decrypted with your password so the database is in RAM (not saved anywhere unencrypted) as long as the browser is open or you click to "logout".
You can export/import all the entries with a simple JSON format (for now, simple "name"+"secret" for each entry). You should encrypt this export file or save it in a encrypted volume yourself.