Yes, the next version of mIRC will use the new Vista method for detecting the default browser. Unfortunately, the current release of Firefox 184.108.40.206 is not Vista compliant which means that it does not set itself as the default browser correctly under Vista. So there is no guarantee that the next mIRC will detect Firefox as your browser.
I am aware of the Firefox 220.127.116.11 issue and am running the 18.104.22.168 RC which fixes this issue; I have confirmed that Firefox is set as the default browser (links in IM programs / HTML files from explorer / etc open in Firefox fine).
As I have mentioned in previous posts however, when I install mIRC 6.21 and Firefox under Vista, mIRC detects Firefox fine for me. You can find discussion of this issue in an earlier Vista-related post.
I did not actually run the mIRC installer, but merely copied my mIRC directory over from a previous Vista build, so perhaps that is affecting things. I will run the mIRC installer and see if that changes things.
Thank you for your input.