• 1 Download Firefox Full Installer For All languages And Systemsv39 {web link}
• 2 Uninstall Firefox from your computer {web link}
  DO NOT remove your profiles.
• 3 After, reboot the computer. Then run the full installer.

Try this:

• Download the Firefox installer and save the file to the desktop
  https://www.mozilla.org/en-US/firefox/all/
• Start the installation with a double-click on the desktop icon

If you keep having problems then it is possible that security (anti-virus) software is interfering. Try to disable your security (anti-virus) software temporarily to see if that makes a difference.

Tried this half dozen times already, no change.

Are you getting any kind of message as to why?

Start your Computer in safe mode with networking. Then start the installer.

Starting The Computer In Safe Mode;
Free Online Encyclopedia

You may have ad / mal-ware. Further information can be found in the Troubleshoot Firefox issues caused by malware article.

Run most or all of the listed malware scanners. Each works differently. If one program misses something, another may pick it up.

Boot the computer in Windows Safe Mode with network support (press F8 on the boot screen) to see if that allows to install Firefox.

• http://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/