I noticed today that when you logout from your cloud based FortiClient EMS instance and then try to login again you receive the following error message in Firefox:
{"result": {"retval": 0, "message": "Local signin is not available in EMS Cloud"}}
It appears to be a cookie related issue in Firefox. When I delete any cookies in the browser referencing "forticlient" I am able to login normally.
Also, this appears to be limited to Firefox as Chrome works fine, even without messing with the cookies.