And it looks like MS are trying to force users to use Edge now, ignoring your current default browser when upgrading. FF are very upset..
The Register
"Internet Explorer users were warned that
Edge, Microsoft's Chrome-chasing new browser, would be the default in Windows 10. But as it turns out, users of Google Chrome, Mozilla Firefox, and other browsers, have found their default web clients switched to Edge after upgrading their PCs to Windows 10.
"It's still possible to switch the application that handles web URLs by default to a different browser. But Beard says the new UI for doing this in Windows 10 makes it less likely that customers will be able to choose the browser they want."