When this had happened to me I had to download all the files in ASCII then delete all those files leaving the blank folder and then re-upload all the files back to the server in ASCII and set the permissions. I refreshed the page and it worked perfectly.

indeed - I am not sure how FF uploads files - I would always recommend using a dedicated ftp package, that you can fully control.

Also expand using something like winzip or winrar to ensure nothing is corrupted during expansion.