The downloads folder does not save files when they are downloaded.
problem particularlly with binary files e.g. adobe reader 10
NIS has been deactivated and problem persists.
Initially downloads are canceled. When retry option is selected file downloads in window but sub directory shows file while downloading and then it is deleted
See What to do if you can't download or save files.
None of the information at that link is specifically related to this issue, it's just general help on downloading files.
When you posted a reply to this thread recently, you were replying to a question that was asked over a year ago.
This thread was locked because of a new feature just added to the Support Forum, that automatically locks a thread if it was originally created over 180 days in the past.
If you need help with Firefox you should ask a new question. You can use this link as a starting point: https://support.mozilla.org/questions/new