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

Firefox Mac OS X 10.9, 10.10 and 10.11 users move to Extended Support Release

Firefox 78 is the last supported version. You can download the latest Firefox 78esr installer for macOS 10.11 using one of these links: https://download.mozilla.org/?product=firefox-esr-latest-ssl&os=osx&lang=en-US https://www.mozilla.org/en-US/firefox/all/#product-desktop-esr

You need Firefox 78.15.0 ESR for Mac OS X 10.11

Firefox 78.15.0 ESR:

• https://www.mozilla.org/en-US/firefox/all/#product-desktop-esr
• https://www.mozilla.org/en-US/firefox/78.15.0/system-requirements/
• https://support.mozilla.org/en-US/products/firefox-enterprise/whats-new-firefox-enterprise

Note that the current Firefox 78.15.0 is the last version for your Mac OS X platform (mozilla-mac-eol-esr1), so you will no longer receive updates.

• https://support.mozilla.org/en-US/kb/firefox-mac-osx-users-esr
• https://support.mozilla.org/en-US/kb/how-download-and-install-firefox-mac