I have made another change that seems to resolve this issue.
Perfect! Thank you
If I then open the mIRC Options->Connect->Options->SSL dialog and load UTN-USERFirst-Hardware.pem (exported from the Windows certificates dialog and converted from DER to PEM format) as the Trusted Authorities file and then connect again to the server, mIRC does not display the warning dialog and connects without any issues, indicating that it was able to validate the certificate.
To avoid exporting certificates by hand one can, for example, use Mozilla's certdata file http://mxr.mozilla.org/seamonkey/source/security/nss/lib/ckfw/builtins/certdata.txt?raw=1
, which is conveniently converted to PEM format by the kind folks at http://curl.haxx.se/docs/caextract.html
Maybe mIRC can ship this file to ease life of its SSL-savvy users? It seems that MPL only applies to the file (and not to the product shipping it), but IANAL.