I can successfully connect to the FTP (upload.comcast.net) server from my home PC, where I have your cable access. However, whenever I attempt to connect at work using the same software (WS_FTP) and same configuration, it states that the server rejected the connection. As it just so happens, we have Comcast high-speed commercial cable access here at work, so I am unsure whether it has to be done solely from my home location, or if there is a way to remedy this. I would like to manage files in my online storage, but I can only upload and download without the use of an FTP client.
As a secondary test, I tried using Internet Explorer for FTP access here at work as well, and it won't connect, giving the error message: "A connection with the server could not be established."
I can at least verify that you do not need to be on the Comcast residential network to be able to access upload.comcast.net. As long as you have a valid username and password, you should be able to connect.
Are you behind a corporate firewall that may be blocking your access? You might try contacting your IT person for more info...