mIRC can use pretty much any port you want. I've sent files using both the POP3 and SMTP e-mail ports before. It doesn't really matter as long as the port isn't being blocked.
Sending files is usually where you find ports being blocked. *YOU* choose what ports you send on in mIRC's options. Alt-O > Connect > Options > Advanced. When downloading, the files don't necessarily come in on the port(s) you choose in mIRC's options. And in most cases, you don't need to worry about that anyhow. Usually, downloads aren't blocked by a firewall. They can be, but they aren't usually except in certain colleges or businesses... some of which don't block all downloads, but only specific extensions, such as MP3.
When testing to see if you can get DCC working, I would disable Windows Firewall and not worry about settings. Once you know that DCC works, then you can turn on Windows Firewall (if you really want to use that as your firewall... there are better ones out there), and see if it works. If not, then you can play around with those settings.
