I can understand how a repetitive FF ad could get on your nerves.
If it bugs you that much, just put the following entry in your HOSTS file:
127.0.0.1
www.factoryfive.com
Your HOSTS file will be found in the Windows\System32\\Drivers\Etc directory. Of course, doing that will also hamper surfing to the Factory Five web site as well, but if you never go there it won't really matter.