Firefox deleting history and data for hotmail, when closing a tab but not the browser

I have used Firefox to log in to Hotmail.com for years. Suddenly Firefox is deleting login data when I close the tab, but not the browser. And when I return to hotmail in a new tab, I now get a window that says it "can't sign me in, try again later." I have to close Firefox and reopen in order to get a new login page. I have always had Firefox set to delete cookies and site data when I close it. Under History setting, it's "use custom settings", "delete history on closing." Under those settings I have browsing and download history unchecked. My hotmail setting is to stay logged in. What's up?! I've always been able to return to a logged in hotmail all day no matter how many times I close the tab and return to it. I've never had that problem before and don't have it with Safari. I also do not have that issue with yahoo mail.