Great support here, Mozilla.

Step one: How do you completely turn off update checks in Firefox v64? You can use a policies.json file to disable updates for all users. https://github.com/mozilla/policy-templates/blob/master/README.md or https://winaero.com/blog/disable-updates-firefox-63-above/

You can use a policies.json file in the distribution folder in the Firefox installation folder instead as also listed in the article, so no need to use GPO keys in the Windows Registry. • https://support.mozilla.org/en-US/kb/customizing-firefox-using-policiesjson

Step two: Note: Some versions of Quantum change how the data files are set up, so they may not be backward compatible.

Install Older Version Of Firefox {web link} Be sure to read everything here.

If you still want to downgrade, look under; I still want to downgrade. Click the Directory of other versions and languages link. Look for the directory of the version that you want. But, remember that old versions may have security issues.

You should also check out Firefox; Extended Support Release {web link} ESR Notes System Requirements