Try temporarily disabling your Firewall on-access/real-time scan while downloading and/or temporarily resetting your Windows Security Zone Policy settings while downloading. Remember to reset them after your download is complete. See the following articles for more information about these steps and and other things to try: https://support.mozilla.com/en-US/kb/unable+to+download+or+save+files http://kb.mozillazine.org/Unable_to_save_or_download_files

You can also disable Firefox using your anti-virus program to scan the downloaded file(s) when downloading is complete: -type about:config in the URL/address bar and press the Enter key -if you see a warning, accept it (promise to be careful) -Filter = browser.download.manager.scanWhenDone -the default "Value" is true -right-click on the pref in the window below "Filter:" and choose "Toggle" -the line should now be in bold, show user set and a value of false -restart Firefox (Exit > Restart Firefox)

See: http://kb.mozillazine.org/Browser.download.manager.scanWhenDone

It happens in safe mode. It happens with virus scanning disabled. Never happens with Safari.

Any other information that you would like to share before someone tries to help you?

It's my first time posting online and I should have been more clear in the original post. I tried everything in "Unable to download or save files." I even did a clean reinstall of Firefox, deleting my personal data.

The same files download just fine with Safari.