Check that you are using an admin account and not a limited account. If your caps lock has jammed it may be difficult to login. Try to make sure no unnecessary applications are running when you attempt to upgrade, and also ensure Firefox itself is properly closed and has ceased, Also say what security software software and firewalls you are using, maybe that is blocking downloads or changes to the computer. For more ideas and solutions have a read of the articles *[[software update failed]] *[[installing firefox on windows]] ( I am assuming you have succeeded in downloading and then finding on your computer the Firefox install file )