scroll to the ‘about’ section and tap on the Version number 5 times (This will enable debug mode)
scroll back to the top and sign out of the Mozilla account (if needed), then navigate to Advanced settings and set the server urls:

FxA url: https://accounts.firefox.com
Token server: https://server_url

Go back to main settings and connect account, scan QR code, and have totp code ready to enter quickly (too slow and the ios app will terminate connection)