Installation won't complete, but Firefox can't be uninstalled, because installation isn't complete.

Take care when following instructions do NOT use any option to remove personal information,because it will destroy your bookmarks and passwords etc.

Have you got security related software of any sort installed

• if so what
• could that be blocking the install

See also

• How to fix the Update Failed error message when updating Firefox
• and as a last resort follow this advice to stop the upgrade process and try again (an old article but info appears up-to-date) https://support.mozilla.org/en-US/kb/Updates%20reported%20when%20running%20newest%20version#w_delete-update-configuration-files
  • based on http://kb.mozillazine.org/Updates_reported_when_running_newest_version

Try to run the Firefox program once as Administrator (right-click: Run as Administrator).

Try to boot the computer in Windows Safe mode with network support (press F8 on the boot screen) as a test to see if that helps.

Here, I have a solution (from Windows 8). In the "Start" screen, right click on the Firefox app. Then click the "open file location" at the bottom of your screen. Highlight and delete all Mozilla files. THEN you can do a fresh download from firefox.com. (Here's what didn't help: starting in safe mode, uninstalling the software, running as Administrator.) Good luck!

Try to run the Firefox program once as Administrator (right-click: Run as Administrator).