You can try downloading the full, offline installer from https://www.mozilla.org/en-US/firefox/all/#product-desktop-release and save it to an easy-to-find location such as your desktop, then reboot the computer before you run the Firefox Setup file. See [[Custom installation of Firefox on Windows]] for install options. If that doesn't work then try disabling any antivirus software before running the installer, as suggested in [https://support.mozilla.org/en-US/questions/1222493 this earlier question]. For general Firefox installation issues, read [[How to install Firefox on Windows#w_having-problems]].