Shoutbox

I downloaded Messenger Plus! Live quite a while ago, and I'm pretty sure I downloaded it for a particular reason. I use firefox and principally use MSN to check my email as it is on the same account. I have always found it annoying that MSN opens my email in IE when firefox is my default browser. I am am sure I downloaded  Messenger Plus! Live because there was a feature enabling you to change which browser is used to open the email, but have yet been unable to figure out how to enable this feature (if it actually exists). If anyone could help me out that would be greatly appreciated. Sorry if this question already exists in other threads.