The installer might be crashing and hanging in memory. If the installer you downloaded has the word "stub" in the name or is not a very large file, try downloading the full installer from this page (scroll down to your preferred language): https://www.mozilla.org/firefox/all/ Sometimes after numerous installs or install attempts the Windows Installer seems to become a bit constipated. You can shut down Windows and start it up again to clear any backlog of deferred actions (i.e., actions that can only be taken when certain files are not in use).