Hi Andrew,

I'm sorry you are having this problem. Can you try downloading the file again from http://www.firefox.com? The file you downloaded may not have been completed.

It is possible that your anti-virus software is corrupting the downloaded files or otherwise interfering with downloading files by Firefox. You can try to disable the real-time (live) scanning of files in your anti-virus software temporarily to see if that makes downloading work. Do not forget to re-enable the scanning in your anti-virus software after you are finished with downloading and installing. .......... See http://kb.mozillazine.org/Unable_to_save_or_download_files (Disable virus scanning in Firefox preferences - Windows)

I also saw the issue, but then downloaded from mozilla.com and it was fine