I'd say it's an issue with the browser(s) not setting the registry value(s) correctly more than a problem with mIRC. Considering this is in regards to Vista and not pre-Vista, I'd suggest bringing it to the attention of Firefox devs so they can find out why it's not setting the registry correctly and perhaps fix it. As for IE, I don't know that letting M$ know will get anything done on their end, but you could also try that.
Khaled, maybe it would be helpful for people who have default browser problems if you could post the location of the default browser setting in the registry as a sticky. Then, anyone who feels like editing the registry (warn them of the danger) can change it to be the correct browser.
