I think this is the nicest example yet of how much the structure of sentences can vary between the two languages, yet it still just makes sense. There's the plural form of "the", the collective noun discrepancy, the word order difference with possessions, the adjective changing to match its noun's count and gender... It's reassuring and enlightening to stop and appreciate what we've learnt in such a short amount of time. "I mobili di mia madre sono bianchi." --> "The furniture of my mother is white." --> "My mother's furniture is white."
I know the feeling! Try reporting an apparently incorrect marking. It's very rewarding when they change the system because of what you tell them as you may well have already found out. I'm not sure about this one though - "pieces of furniture" to my mind would need "pezzi di mobili" but I note that Google Translate (trusted friend - not!) gives "mobili" in Italian when you put in "pieces of furniture."
In English, furniture is an uncountable object - or collective as others have said. Being uncountable, "furniture" implies plural by itself and requires some external unit for counting (such as "one piece of furniture", "four pieces of furniture"). There are other uncountable objects in English like money, research and advice.