What is your operating system & version?
What is your web browser?
It's happening on Windows 7 Home and Windows 7 Enterprise. I'm running Firefox 15.0.1. Problem seems to be wider spread. I tried downloading Firefox 16 and installing it. It also showed as "Publisher Unknown" I tried running an Firefox setup program that was downloaded months ago and it correctly identified the Publisher as "Mozilla Corporation".
I am thinking that there is something in my router/firewall that is failing to identify the publisher certificate. However I don't really understand when it checks the certificate. Is seems to check at download time, wheresas I would have thought it checked at execution time.
It sounds like you are having an issue with certificates on your system, espeically if Firefox 16 also failed. Could you try downloading this installer to see if it also refuses to run? Finally, do you happen to be behind a proxy server?