Sorry, if you are using the standard installer which has to connect back to Mozilla for more files, some security software may interfere. Could you try downloading a complete installer from the following page: https://www.mozilla.org/firefox/all/ Scroll down to your preferred language. If you haven't intentionally installed the 64-bit version before and are considering switching, please see this article first: [[How to switch from 32-bit to 64-bit Firefox]]. Otherwise, you can stick with the standard Windows release. Note: Sometimes the Windows installer gets a lot of deferred updates backed up, and you need to shut down Windows (gently, using the menu) and start it up again to clear the queue before installers work normally.