Roling back normally requires that you get the full installer of the version you want.

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 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

You can use the ManualAppUpdateOnly policy in Firefox 87+ to enable only manual updates e.g. via "Help -> About Firefox".

• https://github.com/mozilla/policy-templates#manualappupdateonly
• https://support.mozilla.org/en-US/kb/customizing-firefox-using-policiesjson

Note that it is important to stay up to date and install the latest security patches.

• https://www.mozilla.org/security/known-vulnerabilities/firefox.html

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.

Note: If you know what version you want, press <Control> (Mac=<Command>) F to open the search bar. Then enter the version number to see the available files.

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

I just installed the 'Mailcheck' version (mailcom_Firefox_Setup_64.exe)...

What is that? Do you want the standard version of Firefox instead?

Thanks for input all!! Apparently, it's actually just an 'add on extension' for Mail.com, who I use for primary email... I finally figured that out and disabled it.

That was very good work. Well Done. Please flag your last post as Solved Problem as this can help others with similar problems. Go to that post and click the 'Solved' button to its right.