-did you try a new d/l and install of 6.14?
-did you double check to be sure that the ports you want are correct in new version of mIRC?
-are you using any scripts/addons that might not be compatible with later versions of mIRC?
-its of course your choice what version of mIRC you use, but be aware that "firewall and other internet security devices" wont protect you from everything. For example, they wouldnt protect you from a dcc exploit that was possible in older versions.
When the majority of ppl dont have any problem dccing with v6.14, that indicates the problem isnt specifically within mIRC as downloaded. the first thing i would try is to d/l a new copy of v6.14 and install it to a clean directory without any scripts and try dcc sending to a friend.