What's the reason for the old version of Firefox? https://www.mozilla.org/en-US/firefox/all/#product-desktop-release

В чем причина старой версии Firefox?

I need this particular version.

Download and install the current version to a different folder from your old version.

Did you try the old Firefox in Safe Mode? https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode#search104

I'm going to download the same version and see what happens.

If you're running the pro version of Windows you can use Windows Sandox VM to run the current version.