Firefox sometimes has trouble finding the "Validate" button on ObjectBinding test pages when testing validation errors, which leads to flaky tests. Coypu times out, claiming that the button cannot be found. This is quite possibly the same underlying problem as in (Once this is confirmed, pleas link this as a duplicating issue).
A workaround based on a retry loop has been added on 23.03.2020. Why Coypu's retry and timeout mechanism cannot locate the button even after waiting is unclear.