Hi
It worked for me without issue. While it's important to change the browser setting in both Firefox and Safari, not doing so has no effect on the browser setting sticking in Safari.
In the Finder: Home>Library>Preferences folder: Move to the desktop the
com.apple.recentitems.plist file, then restart Safari and reset the default browser setting. Restart the computer etc.
If that doesn't work, in the same Library>Preferences folder, move to the desktop the
com.apple.LaunchServices.plist file. Then, restart Safari, set the browser, etc.
Message was edited by: Hawaiian_Starman