Firefox should rename the .part file extension after the download has finished to give the file the final name.
If that isn't happening then your anti-virus software may be causing problems.

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

Thanks for the help. I disabled real-time scanning then tried to download a pdf file. It did not work. Any other suggestions?