Do it exactly as it's stated here: "Close firefox, press ctrl+alt+delete and cancel the firefox.exe process if it's still running. Then go to start --> Run and type "firefox.exe -p" without the quotes. If that doesn't launch Profile Manager try "firefox.exe -ProfileManager". If you see any other profile apart from default, delete it." So make sure you are typing firefox.exe -p or firefox.exe -ProfileManager, not just firefox.exe
