Pesquisando no google, eu encontrei esta página no Firefox páginas de suporte:
Choose a valid download folder in Safari Preferences
If you receive the error [filename] could not be saved, because an unknown error occurred, your problem might be caused by an interaction with the Safari browser and your operating system.
To fix the problem, open Safari's Preferences and change the Save downloaded files to: setting to a valid folder (such as your Desktop). Then restart Firefox.
Isso pode ajudar você a corrigir seu problema com o Firefox.