I was curious about this one, as I have the faintest memory of setting my DCC Ports to 80/80 in order to defeat someone's corporate firewall restrictions. I can't really tell if this is some drug induced memory due to the volume of Nyquil and Theraflu I have consumed in the past 48 hours, so I did the research myself.
I already have every version of mIRC from 2.1a to current installed just for this reason, and now "Myth Busters mIRC" will determine if Toad's recollection is as accurate as he hopes.
Starting from mIRC v6.12 and working back, I went into the DCC > Options dialog and configured the High and Low port ranges to both equal 80. I ended up testing back 22 versions to mIRC v5.3 released December 13, 1997, as this is the first version to add a user-definable port range in the DCC options.
The results... when we return, in a moment.