I opened Firefox today, it did an update before loading and when I opened my hotmail.com all the messages in my Focused folder were gone including one I needed to respond to today. There was picture of a hot air balloon and a message saying "All done for the day. Hope you enjoy your empty Inbox". I checked the Other tab, the Deleted and Junk folders but nothing from my Inbox was there.
Firefox is a Browser not a email program. Hotmail issue is for hotmail support resolve and fix since Firefox has nothing to do with hotmail.
Can you check your email account in the Edge browser to see what is going on there? It's possible Firefox has a different view of your mail, but I think the first step is to confirm that your mail is still there, using Microsoft's own browser.