How would Firefox know where to save that specific file you uploaded to the server ? If you would want to save the file in the same folder then you will have to select that folder yourself. The file will either be saved in the default download folder with a possible number appended to avoid replacing the file or Firefox will ask you for the location where to save the file and ask for confirmation whether to overwrite. Firefox can remember where you save specific files (file extension) from a website via the Site Preferences (won't work in private browsing).