I deleted my account using another iMac. Then deleted the account on my other iMac. Went back to original iMac and created new account and was able to sign in. When I went to other iMac and clicked set up sync I entered the correct user name and the correct password. The sign on logo rotated a few times and just stayed at sign on window. I have tried to log on in safe mode same thing. No matter what I do I can't sign on. Also sometimes the sync option just shows a blank box on the screen. Once that happens I have to restart Firefox.