If your default browser is IE, any link you double-click on in mIRC will open in a new window rather than a new tab.
By contrast, opening links from outlook, word, mumble, or even doing start > run > "http://www.google.com" all open in a new tab
the only way I can replicate this behaviour manually is by doing start > run > "iexplore.exe http://www.google.com" - if this is what mIRC is doing it's The Wrong Thing - it should just open the URL and let the OS work out what the browser is (as per
http://support.microsoft.com/kb/307382 or similar)
