Hmm If you have a current Firefox installed on your system and want the old Firefox 83.0 just for this one thing then perhaps you should use the portable Firefox 83.0 version. The portable Firefox uses the official Firefox version but with a custom launcher that keeps the Firefox and Profile self contained so it will not touch your current Firefox install on Windows. You can also run this completely off a usb flash drive for example. https://sourceforge.net/projects/portableapps/files/Mozilla%20Firefox,%20Portable%20Ed./Mozilla%20Firefox,%20Portable%20Edition%2083.0/ and pick your language.