Firefox shouldn't have anything to do with extracting from Zip folders. When you've downloaded the Zip folder, go into the folder it is in, right-click on it and click "Extract files to..." or whatever it says (I have WinRAR so it says something slightly different to the default Windows thing). Then it should either extract the files, or give you an error message if the Zip folder is corrupt.
Firefox shouldn't play any part in extracting the Zip files.