I got the same offer from Sussex for International Development 😄 I think Sussex are trying to avoid top students placing them as insurance by only giving the lower offer if they're firmed, I reckon that really they are just looking for ABB so you might be okay putting them as insurance. However, if come results you got ABB and your course is already full at Sussex then you are not guaranteed a place. It's ultimately up to you but I would recommend putting your genuine favourite uni as your firm regardless of grades and perhaps put Liverpool down as the insurance if you think there's a fair chance you'll miss your firm offer to avoid risking it.