I have 3 different Windows 7 PCs that are now exhibiting the same problem. All started this week with version 54 update. I cannot get Firefox 54 set as the default browser. It will ask to set as default every time firefox is started. I was running McAfee EPS but have completely un-installed and McAfee support now indicates this isnt' caused by McAfee since it isn't deployed. No other security software is installed on any of the machines effected. I have tried un-installed in normal mode and in safe mode. I've tried deleting all profiles and restarting. I've tried manually deleting all Mozilla folders on all user profiles after the un-installs (many different folders and locations found). I've run ccleaner after the un-installs to ensure things are cleaned up. I've rebooted before the re-installs. I've always run install with 'run as administrator'. The user accounts already have local admin level security. Firefox does NOT show up in default programs (see attached image). It hasn't starting this week when this problem started after v54 starting getting updated. I am typing this question in Firefox 54 and it does function. I've been searching and searching for other things to try but don't understand what is happening that Firefox available to be set as a default. Please help provide a solution as this is really causing productivity issues and I really don't want to migrate to Chrome.