It is possible that your anti-virus software is corrupting the downloaded files or otherwise interfering with downloading files.
Try to disable the real-time (live) scanning of files in your anti-virus software temporarily to see if that makes downloading work.

See "Disable virus scanning in Firefox preferences - Windows"

• http://kb.mozillazine.org/Unable_to_save_or_download_files

If you have that problem in the Download Manager window then reset the browser.download.manager.retention pref to 2 on the about:config page via the right-click context menu.

• http://kb.mozillazine.org/about:config

I have this problem too, I have selected the downloads folder, when the download is finished it disappears, I have done a search of the files, and they simply are no longer on my computer. How do I fix this. I looked at the above articles and all that was suggested was to change the strings, but didn't say what to change the strings too. I have already deleted firefox and then redownloaded it and reinstalled it. How do you fix this?

I have reset everything to default, I have eliminated cleared the database, cleaned the registry, reinstalled and the files still disappear after they are downloaded

I have done the steps here: http://kb.mozillazine.org/about:config

and this one and still nothing: http://kb.mozillazine.org/Unable_to_save_or_download_files

I have this issue to, in Window Vista with FF 18.0.2; if have disabled plug-ins, disable Avast firewall and scanning, reset Firefox to default settings. The only thing I have not done is reinstalled because I cannot download the file. Any thoughts