I'm not sure it's possible. This article doesn't hold out much hope: Why can't I switch back to Hotmail? - Microsoft Windows Help.

This isn't a Firefox issue. Microsoft migrated Hotmail accounts to Outlook. So you may want to ask in a Microsoft forum.

What I do is to use Firefox and my e-mail program separately. Currently, Microsoft Hotmail is using a program called Windows Live Mail. The messages are displayed in a nice large window. If you decide on Hotmail, check out; https://addons.mozilla.org/en-US/firefox/ to find an add-on to let you know you got mail.