Ok lets try to break this down for you.
First off, i had this problem when i started using vista, so i did a search on here and got a solution from this thread.
When you use Firefox for the first time it asks you if you'd like this to be your default browser, Yes? when you click yes and click a link that's in mIRC, IE still opens! That's the problem right there, None of this going to control panel and settings etc, That's not what this thread is about. Yes we know (as you explained) Is a kind of fix, But it's not what the issue is about mIRC is still using IE even when you've clicked 'yes' to Firefox being the default browser.
And if you post a reply saying 'i've told you the solution' again, i hope this thread gets locked, as you are clearly missing the point to it. nuff said