First: 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

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

Relatively the same issue. Color options for the OLD 'menu bar' and 'tabs' were just fine for my eyesight. Now someone at Mozilla decided to change those colors options to what THEY like and have eliminated the OLD options. Version 85 colors were adequate. New and fresh isn't what I was looking for. AT LEAST bring back the OLD (v85) color options. Is there a way to roll back to v85? Yeah, yeah. The whole world is filled with security issues but I really doubt changing color options will make anyone safer.