Hi, Please try right-clicking on the Firefox installer and '''Run as administrator'''. If the problem persists, you can try to include the installer and its processes as fully trusted/allowed in any security software before starting the installation. You can also try to temporarily disable the security software for a few minutes. In this case you may also have to include the newly installed Firefox + sub processes in the security software configuration before starting Firefox. You can also try [http://portableapps.com/apps/internet/firefox_portable portable Firefox]. [http://portableapps.com/support/firefox_portable Support]