If want the xsd for the e-fff, you must user the version of the UBL 2.0 . This is the correct version: http://docs.oasis-open.org/ubl/os-UBL-2.0/xsd/
Mind that your xsd validation is only going to verify the format of the xml file such as
- are all the required fields present ?
- are the fields in the correct order ?
- are they respecting the cardinality?
An xsd validation does not validate the business rules. that is why we also provide the different business test scenario’s for you to test.