Thanks for the information. Like most users, I use the most recent stable public release of Firefox which is Firefox 2.0. That is the version that I test mIRC with.
I'll read up about the UrlAssociations key under Vista, however if I make mIRC only use that key to determine the default browser it will end up not working with any version of Firefox other than the trunk release since, as you say, all versions of Firefox other than the trunk release set the default browser key incorrectly under Vista.
Once a new stable public version of Firefox is released I will take a look at what it's doing (assuming it doesn't change the method by then) and will try to find a way to cater for both the new and old versions of Firefox.
