New Yahoo! Mail defaults to IE

I recently updated from the old version of Yahoo mail to the most recent. When I open it defaults to Internet Explorer. How can I set the default browser to Mozilla Firefox?
This does not happen when I open via Yahoo messenger icon , nor when I use the old Yahoo!Mail

http://uk.rdyahoo.com/customiz e/links/ymsgr7/*http://mail.yahoo.com/?.intl=uk

It is when I use the new C:\WINDOWS\SYSTEM32\RUNDLL32.EXE C:\PROGRA~1\Yahoo!\Common\ymmapi.dll,OpenURL http://mail.yahoo.com/?.intl=uk&.redir=ymmapi9

I see the old location is C:\Documents and settings\All Users\Desktop whereas the new location is my personal desktop.

Go to Start/Set program access and defaults/-

Make Firefox the default browser and Yahoo default email.

You can also get a Yahoo addon for Firefox.

Firefox is default browser & I have Thunderbird as default email. I will post screenshots & further details soon.
Thanks for suggestion. Tried but problem persists

You talking about apples and oranges here:

Firefox (Mozilla) is a browser. To make it the default browser> Tools> Main> System Defaults> click to CHECK 'make Firefox the default', then 'Check now'> Yes.

To update the mail: Control Panel> Internet Options> Programs> change the email carrier to Thunderbird> uncheck 'IE should check to see if it's default browser'. (IE is pushy, so be sure this is UNCHECKED)

When you access Yahoo Mail through a URL, you're going through your browser. When you create a shortcut to Yahoo Mail, you're skipping the 'middleman' so to speak