Windows single sign-on (SSO) is a new Firefox feature that allows you to log in to Microsoft, work, and school accounts using credentials from your Windows 10 operating system. To enable it, click the Firefox menu button, click, select on the left, go down to the Logins and Passwords section and check the box next to the Allow Windows single sign-on ... setting.
To see if you have any accounts configured, enter users in the Windows taskbar search field, click on Add, edit, or remove other users and click on Email & accounts. You'll see a window that looks like this:
If you have accounts in the Accounts used by other apps section, Firefox will use that information to log you in to Microsoft sites including Outlook and Office 365, as well as any work or school accounts that use Microsoft authentication.