How do I get MSN MAIL to open in FIREFOX BROWSER?

Some reason now my email wont automatically open in firefox when I am logged into windows live. It started to open in INTERNET EXPLORER. I do not know hwy it does this now as before it always opened in Firefox. How can I change it back. My default browser is already set as FIREFOX. I did upgrade to newest version IE last few weeks but I typed in when it was installing that I did not want to make IE my default browser.