What is your computer system?

Please explain the problem in detail. What happens? What is/are the exact error message(s) ?

All you should need to do is to download the full installer and run it. Download Firefox For All languages And Systems {web link}

It is possible that (anti-virus) security software is causing the problem. Try to disable security software temporarily to see if that makes a difference.

Boot the computer in Windows Safe Mode with network support to see if that has effect.

• https://www.bleepingcomputer.com/tuto.../how-to-start-windows-in-safe-mode/
• https://www.digitalcitizen.life/4-ways-boot-safe-mode-windows-10/